Hello venerable,
It seems like the view that this is a Mahasangika text is not necessarily solid. Lin, the author of the study I linked notes that several Chinese sources place this as a Darstantika/Sautrantika influenced text and say that Harivarman was a student of the famous Kumaralata, founder of the sect/sub-sect.
Moreover, according to Xuanchang’s biography, Harivarman was a student of a Sarvāstivāda master named *Kumāralāta (究摩羅陀).18 This *Kumāralāta may be the teacher identified as a Dārṣṭāntika who was also later considered the first “Sautrāntika. (Mind in dispute, pg 14)
The biography of this person is interested, even if it is hard to verify:
Xuanchang’s biography24 provides the following information about Harivarman’s life. He was born in central India (中天竺) in a Brahmin family (婆羅門子). In his youth, he learned the Vedas and other sciences, and later he was ordained in the Sarvāstivāda order (薩婆多部) and became a student of the “monk of doctrine” (*dharmaśramaṇa 達摩沙門) *Kumāralāta (究摩羅陀). *Kumāralāta taught him the “great Abhidharma of Kātyāyana (迦旃延) with thousands of gāthās,” probably the Sarvāstivāda Abhidharma text Jñānaprasthāna. Having fully learned this work, Harivarman was unsatisfied and disillusioned with Abhidharma. He then spent several years studying the entire Tripiṭaka and traced all the different teachings of the “five sects” (五部) and “nine branches” (九流 *srotas) 25 back to their common origin. Thereafter, he engaged in debates with other Buddhist teachers and tried to persuade them to return to the original Buddhist teachings. Those teachers were reluctant to abandon their sectarian doctrines. As a result, Harivarman became unpopular among them. However, the Mahāsāṅghikas (僧祇部) in
the city Pāṭaliputra (巴連弗邑), who also claimed that their doctrines were the origin of the “five sects,” heard about Harivarman and invited him to live with them. There Harivarman studied Mahāyāna (方等 vaipulya) and the teachings of all traditions. He wrote the *Tattvasiddhi, in which he investigated and criticized the different doctrines from various traditions, especially Kātyāyana’s Abhidharma system. Harivarman’s stated purpose in writing this work was to “eliminate confusion and abandon the later developments, with the hope of returning to the origin” (除繁棄末慕存歸本). The biography concludes with a record of Harivarman’s victorious debate with a Vaiśeṣika teacher, from which he earned a great reputation.
But he also notes:
However, in my reading of the section on mind in the TatSid, I do not find any explicit statement by Harivarman that rejects the authority of Abhidharma, and he even agrees with the Ābhidharmikas, or teachers who are explicitly affiliated with a certain Abhidharma tradition, on some doctrinal points. This indicates that we may not simply label Harivarman as a “Dārṣṭāntika” or a “Sautrāntika.”
He also quotes the Chinese master Jizang :
Some say that [he] chooses and follows those who are right and
records whoever is superior. [He] discards those inferior [teachings] of different teachers and adopts the superior [teachings] from different sects.
Others say that though he rejects all different [teachings], he mainly adopts
[the teachings of] the Dharmaguptaka sect (曇無德部).
Others say that [he] criticizes extensively the Abhidharma,32 and agrees
specifically with the Dārṣṭāntikas. The Tripiṭaka master Paramārtha (真諦三㯿) says that [Harivarman] adopts the meanings of the Sautrāntikas (經部). When the Abhidharmakośa is examined, those meanings attributed to the Sautrāntika sect mostly agree with the *Tattvasiddhi.
Seems like the Chinese were just as confused! Modern scholars too!
Fukuhara (Fukuhara 1969: 25-52) examines all the accounts and finds that the TatSid has doctrinal positions shared with all of the schools mentioned. His conclusion that Harivarman was a Bahuśrutīya is mainly based on the account from the translator Paramārtha (真諦 499-569 CE), and he believes
that Paramārtha’s account came directly from India and hence must be more reliable (51). Katsura (1974: 29-49) examines the ten points of controversy discussed in the introduction section of the TatSid,35 finds that on these ten issues only the Bahuśrutīyas, the Prajñāptivādins, and the Theravādins have no disagreement with Harivarman. But the Prajñāptivādins and the Theravādins have issues other than these ten that disagree with Harivarman. Thus, the Bahuśrutīya is the only option left.
Lin goes on to further complicate the use of certain school names and associations and assigning them to particular doctrinal positions, noting that things were much more fluid at the time. He notes that ultimately it is impossible to determine the “school affiliation” if such a thing existed back then in a hard sense. He notes that all we know about him is that he is knowledgeable about Sarvastivada and criticizes it from a sutra perspective. Thus it is fair to say he was a Sautrantika, at least in that sense.