Why did Marco Polo say the Buddhist religion originated in Sielan?

Probably at that time period, Buddhism were established in Ceylon at early stage, just because that we can not tell that it is true. Even the places which mentioned in Dharma, can not find here in Ceylon, like Mount Himalaya, Gijjakuta Rock.

Probably because he was told so and never started to investigate the real origin of things? Even today there are many Sinhalese people claiming the origin in the country they live. One may find such similar thoughts in all other countries and one also may find Jesus with balck skin or Chinese eyes in other countries. That's the nature and requirement for all -isms and -ists: Strong Self-identification.

Dhamma and means for liberation should not be dig up be focus on objectification (pacanca) although it is usually refined identification what gives one rise toward interest and can be used of course also strategical wise "What, if I let go of it, would lead to my long term happiness?", thought of "who ever defines one self, limits and suffers with it", especially if not something leading to go beyond.

If Marco Polo ever had met the good Dhamma, real "Buddists", he would not have returned. So it's probably good to investigate travelers who found the way that leads to rest.

“Is it possible, lord, by traveling, to know or see or reach a far end of the cosmos where one does not take birth, age, die, pass away or reappear?”

“I tell you, friend, that it is not possible by traveling to know or see or reach a far end of the cosmos where one does not take birth, age, die, pass away, or reappear.” ... But at the same time, I tell you that there is no making an end of suffering & stress without reaching the end of the cosmos. Yet it is just within this fathom-long body, with its perception & intellect, that I declare that there is the cosmos, the origination of the cosmos, the cessation of the cosmos, and the path of practice leading to the cessation of the cosmos.” Rohitassa Sutta

{Note that this has not been given to keep one on wandering on by trade, exchange, stacks, or use it for such, but for liberation}

To answer a further argument, actually not leading to liberation at all:

"Well, Marco Polo visited India and China along the way too. If he found Buddha existed in India then he would have mentioned it that way instead."

Why should he have traveled around and possible ask everywhere the same question, everybody he meets? If one lives amoung people with a certain believe, then one might be missinformed, since not knowing. As the Dhamma is about seeing your self (to gain liberation), what is not possible of thing which have already decayed (history), it does not support the feed the attention on improper means but to come there where reality happens and why. Once again in other words:

“Is it possible, lord, by researching, to know or see or reach the Buddha or Dhamma, where one does not take birth, age, die, pass away or reappear?”

“I tell you, friend, that it is not possible by research to know or see or reach the Buddha or the Dhamma where one does not take birth, age, die, pass away, or reappear.” ... But at the same time, I tell you that there is no making an end of suffering & stress without researching after the Buddha and the Dhamma. Yet it is just within this fathom-long body, with its perception & intellect, that I declare that there is The Buddha and the Dhamma, the the origination of The Buddha and the Dhamma, the cessation of the Buddha and the Dhamma, and the path of practice leading to the cessation of The Buddha and the Dhamma.”

In regard of places, as the Buddha told, there is no place to be found in the world which is not actually a cementery, and ther is not a singe moment in the world in which no aging, sickness and death takes place. So what does one cling on here? Be wise. Don't waste your time, what ever might have been does not matter on the fact, that a raw possibility to be born as human, possible not stupid (?) and meeting the Dhamma, might be wasted away!