What is a "True Buddhist"?

A Buddhist is someone who takes refuge in the triple gem. You don’t get a badge and no one licenses “True Buddhists” but if someone takes primary refuge in the triple gem with all that entails, then it is enough for that person to be considered Buddhist.

I suppose the uniqueness of the Buddha's Dhamma is realizing not-self & emptiness & knowing Nibbana. The Suttas use the word "true" in relation to certain Dhammas, such as:

For that which is false has a deceptive nature, while that which is true has an undeceptive nature — Nibbana.

Tañhi, bhikkhu, musā yaṁ mosadhammaṁ, taṁ saccaṁ yaṁ amosadhammaṁ nibbānaṁ.

MN 140

As long as my true knowledge and vision about these four noble truths was not fully purified in these three perspectives and twelve aspects, I didn’t announce my supreme perfect awakening in this world with its gods, Māras, and Brahmās, this population with its ascetics and brahmins, its gods and humans.

Yāvakīvañca me, bhikkhave, imesu catūsu ariyasaccesu evaṁ tiparivaṭṭaṁ dvādasākāraṁ yathābhūtaṁ ñāṇadassanaṁ na suvisuddhaṁ ahosi, neva tāvāhaṁ, bhikkhave, sadevake loke samārake sabrahmake sassamaṇabrāhmaṇiyā pajāya sadevamanussāya ‘anuttaraṁ sammāsambodhiṁ abhisambuddho’ti paccaññāsiṁ.

SN 56.11

Therefore I guess Buddhadasa may be saying a True Buddhist is one that has realized the True Dhamma.

I suppose if these & other higher unique Buddhism truths have not been directly seen, whatever Buddhist 'path' one is practicing is probably not much different to other religions. This said, the suttas appear to contain teachings saying a non-Ariya is a disciple of the Buddha, such as:

Inasmuch, young householder, a disciple of the Noble Ones (1) has eradicated the four vices in conduct, (2) inasmuch as he commits no evil action in four ways, (3) inasmuch as he pursues not the six channels for dissipating wealth, he thus, avoiding these fourteen evil things, covers the six quarters, and enters the path leading to victory in both worlds: he is favored in this world and in the world beyond. Upon the dissolution of the body, after death, he is born in a happy heavenly realm.

DN 31

Nobility starts with understanding. As adults, we are so busy with our careers, our spouse and children and chasing after wealth, fame and power. There are times when we would pause for a moment in our worldly pursuits and wonder if this is all there is. When we understand and grew weary with such worldly pursuits and yearn for something more meaningful, that’s when we begin to understand the difference between the ordinary and the noble. Knowing the ordinary is to understand what the world has to offer. Knowing the noble is understand what the mind has to offer.

Nobility begins with seeing reality. When the Buddha saw old age, illness and death, he could no longer bear to live in a façade, in an illusion that nothing bad will happen. When we are young and intoxicated with the vigour of youth, nothing can stand in our way. We may experience personal crisis like the loss of our jobs, the loss of our loved ones, the loss of our reputation, the loss of a marriage and so on. But we can shrug it all off and push ahead with confidence. But as we grew old and come closer to the greatest loss of all, our life; we are not so sure anymore.

Nobility grew with the practice in the Dharma. Struggling to understand, reflect and penetrate the Buddha’s teachings. Doing our regular meditation despite our busy schedules. Seeing the futility of our habitual thoughts, speech and actions in helping us achieve truly meaningful well-being and peace, we begin to change. Strangely, the change in behaviour just happens to align to that described of a Dharma practitioner. It is like we did not mean to visit certain places or attractions. But as we followed the prescribed path, these places just appeared.

We are lucky to be born as Buddhists or to come across Buddhism whether we considered ourselves as true Buddhists or not. In an age where the Buddha’s teachings existed, we have the opportunity to ponder and investigate the Dharma. And if we did make sincere efforts then as we grew older and more mature, many of these teachings began to make sense both logically and experientially. This is when we are genuinely thankful for being Buddhists. As to the question of being a true Buddhist or an Ariyan one that is not important anymore; having peace and well-being within is all that truly matters.

A true Buddhist should at least adhere to the five precepts of Buddhism. They should have faith in the teachings of Buddha and strive to live a peaceful life, free from violence. It is important for a true Buddhist to continually learn and follow Buddha's teachings.

The main teaching of Buddha was the "path of enlightenment". Anyone who follows this path can indeed be considered a true Buddhist.

Buddha once said, "To see my Dharma (understand my teachings) is to truly see me". This is encapsulated in the phrase "yo dhamman passathi so Mang passathi".

One's status as a true Buddhist is not determined by what is written on their birth certificate or National Identity Card (NIC). Rather, it is their understanding and practice of Buddha's teachings that define them as a true Buddhist.

What he is meaning by using the word ariya as a noble person as in a person who has experienced the goal and is actually following the path. A sottapanna onward on the stages of liberation.

He is making the distinction between ordinary beings who are embroiled in the swirl of emotions of the world, Puttajanna, and those who have experienced the goal and are walking the path towards liberation, Ariya. For the latter liberation is secured, for the former they could spend lifetimes practising and bear no security that liberation will be attained.

There are faith followers who follow the precepts or pray to statues or meditate on what they were told who may not be "true Buddhists" where as there are those who have experienced right view, have experienced nibanna and are walking the path to cessation.

Buddhadasa is pointing out the distinction between religious and philosophical practices. Religious practices are more or less passive activities, in which people rely on rituals, symbols, liturgies, the authority and guidance of clergy, the intercession of divine or transcendent masters, or other sorts of revealed or esoteric leadership. Philosophical practices, by contrast, imply a search for understanding through and beyond the superficialities of religious teaching. In specific, he's distinguishing between:

1. Lay Buddhists, who burn incense, chant invocations or teachings, place offerings of food or flowers before statues of the Buddha, do ritualistic bowing, etc., and…
2. Monastic Buddhists, who may or may not do any of those things, but who follow the precepts, study the texts, practice meditation, and generally seek out the true substance of the teachings beyond the mere forms and gestures of lay practice.

This isn't specific to Buddhism, incidentally, but is a truism is every faith. For most people a faith is a mere source of authority which they either accept over their lives or reject. For a few, a faith is a path to understanding that needs to be embraced and absorbed. Buddhadasa is suggesting that embracing the faith is more profound and wholesome than merely accepting it as authoritative and I can't disagree, though I suspect he may be dumbing it down a little for his audience (i.e., presenting the philosophical view in an authoritative tone that lay practitioners might respond to).

Who the "true" Buddhists are, are clearly defined in the sutta below:

“Mendicants, these eight people are worthy of offerings dedicated to the gods, worthy of hospitality, worthy of a religious donation, worthy of greeting with joined palms, and are the supreme field of merit for the world.

What eight? The stream-enterer and the one practicing to realize the fruit of stream-entry. The once-returner and the one practicing to realize the fruit of once-return. The non-returner and the one practicing to realize the fruit of non-return. The perfected one, and the one practicing for perfection.

These are the eight people who are worthy of offerings dedicated to the gods, worthy of hospitality, worthy of a religious donation, worthy of greeting with joined palms, and are the supreme field of merit for the world.

Four practicing the path,
and four established in the fruit.
This is the upright Saṅgha,
with wisdom, ethics, and immersion.

For humans, those merit-seeking creatures,
who sponsor sacrifices,
making worldly merit,
what is given to the Saṅgha is very fruitful.”
AN 8.59