Would zen Buddhists agree with this quote from Chih-I?

I think Zen Buddhists would agree. Tiāntāi had been hugely influential on Japanese Buddhism especially through it's Tendai iteration. Consider: Venerable Shinran was a Tendai seminarian, as were Venerables Nichiren & Eisai. Venerable Hōnen was also ordained as a Tendai priest. As we can see, many later schools/sects of Japanese Buddhism actually sprout from Tendai as their mother.

The above quote is from Mahāśamathavipaśyanā T1911.38.661c-662a.

A lot of this discourse is to-do with identification, suchness vs differentiation, real vs unreal, ultimate vs provisional, nirvāṇic vs worldly.

The book you are quoting from compares Venerable Zhìyǐ's round fusion with the Madhyamaka's teaching. it seems.

This seems relevant in light of that, from the Sinitic recension of Venerable Nāgārjuna's Mūlamadhyamakakārikā:

All Buddhas either speak of self or speak of no self. All dharmas’ true aspect, within this, there is neither self nor no self. All dharmas’ true aspect is defined as mental activity’s and spoken language’s ending. There is no arising and no cessation, there is calm extinction, such is nirvāṇa. All is real, all is unreal, all is both real and unreal, all is neither real nor unreal: this is called all Buddhas’ dharma.

(Āryanāgārjunasya Mūlamadhyamakakārikāyām Ātmaparīkṣā, T1564.23c16 : 諸佛或說我 或說於無我 諸法實相中 無我無非我 [etc.], section 18)

Compare the above with the Venerable Vimalākṣa's commentary on that section, for his or her Mādhyamikaśāstra, one of the three theses of East Asian Madhyamaka (三論宗 Triśāstravāda, "the School of the Three Treatises"), which would have been well-familiar to Venerable Zhìyǐ:

Reply: Those who are attached to dharmas classify dharmas into two kinds, some being worldly, some being of nirvana. They say that the nirvana dharmas are calm and extinct, but do not say that the worldly dharmas are calm and extinct. In this treatise it is taught that all dharmas are empty in nature and have the characteristic of calm extinction. Since those who are attached to dharmas do not understand this, nirvana is used as an example. Just as with your assertion that the characteristic of nirvana is emptiness, with no characteristics, calm extinction, and no vain thoughts, so it is with all worldly dharmas.

Question: If the Buddhas do not teach self, non-self, and the cessation of all mental activities and the cutting-off of ways of verbal expression, how do they make people understand the real character of dharmas?

Reply: All the Buddhas have unlimited powers of skilful means, and dharmas have no fixed characteristics. In order to save all living beings, they may teach that everything is real, or they may teach that everything is unreal, or that everything is both real and unreal or that everything is neither unreal nor not unreal. If you search for a real nature of dharmas, you will find that they all enter into the ultimate meaning and become equal, with identical characteristics, which is to say no characteristics, just like streams of different colour and different tastes entering into a great ocean of one colour and one taste, which is to say no taste. At the time when one has not yet penetrated into the true character of dharmas, each one can be contemplated separately as unreal, existing merely by the combinations of conditions. There are three levels of living beings; superior, average and inferior. The superior person sees that the characteristic of dharmas is that they are neither real nor unreal. The average person sees the characteristics of dharmas as either all real, or all unreal. The inferior man, since his powers of perception are limited, sees the characteristics of dharmas as a little real, and a little unreal, regarding nirvana, because it is an inactive dharma and does not perish as real, and regarding samsara, because it is an active dharma, empty and false, as unreal. Neither unreal nor not unreal is taught in order to negate ‘both real and unreal’.

(Āryavimalākṣasya Mādhyamikaśāstre Ātmaparīkṣāparivartaḥ T1564.24a15: 有人說神。應有二種。若五陰即是神。若離 五陰有神。[etc.], translation Brian Christopher Bocking)

Furthermore, returning to the source text of the commentary above, we later have this, a passage Ven Zhìyǐ would have been very familiar with:

Between nirvāna and this world there is not even a slight disparity. Between this world and nirvāṇa there is also not even a slight disparity. From nirvāṇa’s true apex towards this world’s apex, like this, there are two apices, like this, there is not the smallest sliver of disparity between them.

(Mūlamadhyamakakārikāyām Nirvāṇaparīkṣā, T1564.35c27: 涅槃與世間 無有少分別 世間與涅槃 亦無少分別 涅槃之實際 及與世間際 如是二際者 無毫釐差別, section 25)