https://suttacentral.net/general-guide-sujato
Sujato addresses several claims in his essay, and has some helpful hints on the historical placement of the Buddhadhamma.
The Buddha was not a social reformer - his teaching was about total renunciation of the world. But there are plenty of discourses that reject the birth-based caste system and instead establishes that deeds and actions are the sole criteria for judging a person.
An example: https://suttacentral.net/mn98/en/sujato
You are mistaken about Karma (Pali: Kamma). Kamma is a central pillar in the Buddha's Teaching. It is, arguably, one of the most important aspects, since Nibbana is described as the cessation of kamma.
https://www.accesstoinsight.org/lib/study/kamma.html
Adi Sankara's rejection of the Buddha's Teaching was not a simplicistic dismissal based on sectarian divisions. His rejection was based on philosophical differences - the Vedanta Sutras make that very clear.
In any case, my main point was that it is nonsensical to call the Buddha as a Hindu.
I am mystified as to what you think I’ve got wrong about karma. As you say Nirvana involves the cessation of karma. Jnana alone is its cause. This is the same as Advaita Vedanta (and several other darshanas.). The point of difference is what pragmatic concessions are to be given to those who cannot become monks. To be a Buddhist in ancient India was to be a monk. There were lay people who donated to the monks but they otherwise carried on their ancestral traditions. There was no specifically Buddhist way to be a layman until after the religion left India. In Smartism, to learn Advaita Vedanta and give up karma and become a monk is the highest goal but if you can’t, rituals and worldly life still have some positive worth. Vaishnavism, Shaivism, and Tantra were even more this-world oriented and in actual practice more egalitarian and accessible than Buddhism nevermind whatever rhetoric was put forth.
I don’t understand the distinction you are trying to make between sectarian division and philosophical difference. A philosophical difference is what makes a sect a sect no?
We agree that Buddha was not a Hindu. For exactly the same reasons we must say Buddha was not not a Hindu. The whole question anachronistically projects latter day concerns and concepts too far into the past. But was Buddhism (or is or could it be) Hinduism? That’s the real bone of contention anyway and its a much more ambiguous question. We could mention how Buddha is an avatar of Vishnu or Yoga borrows concepts from Mahayana or the wholesale adoption of Shaiva tantric concepts in Vajrayana. I personally like to compare with Jainism. It is just as old as Buddhism, heretical to Astikas for exactly the same reasons as Buddhism and yet thoroughly integrated into social structures, modes of worship, and literary traditions to the extent that the man in the street is not even aware that Jainism is anything more than another Hindu sect. I think that would have been the fate of Buddhism if it had survived to this day. And that’s the key. It went extinct a long time ago. So the correct answer to my question is: “we don’t know.”
It is troubling to see how the current wave of jingoism coupled with religious fervor is trying to reduce the Buddha to a mere pawn in an imaginary, all-encompassing Hindu pantheon. Maybe you are not deliberately trying to do this, but the muddled and confused efforts of numerous foot-soldiers expose the sad trend all the same.
This is not true, there's no conflict. The Buddha was not interested in abstract doctrines but only in what was most pragmatically useful for attaining stream entry, enlightenment and liberation from craving and desire. People who are actually pursuing these goals, even in modern times (with very compelling results, though obviously any claims to arhat status will always be viewed skeptically by most) have clarified how anatta is entirely compatible with a Higher Self as with Brahman.
https://en.wikipedia.org/wiki/Adi_Shankara
From the second para:
He also explained the key difference between Hinduism and Buddhism, stating that Hinduism asserts "Ātman (Soul, Self) exists", while Buddhism asserts that there is "no Soul, no Self".
For that matter, even Shankara's description of the Atman is entirely in the negative, stating what the Atman is not. So even while accepting the existence of the Atman, he's clearly tending towards a universalist description that, again, is quite compatible with a practical understanding of the Buddhist doctrine of not-Atman.
Buddhists themselves reduced the historical Shakyamuni to one member of a pantheon long ago. This was a clear trend in Mahayana and Vajrayana before Allauddin Khilji came and settled the issue once and for all.
1. There is no reason to believe the Pali Tripitaka represents the authentic words or teachings of the Buddha. He would have spoken Magadhi as his mother tongue. Pali was a Western Prakrit and moreover a trade and literary language not a popular one. Also the Pali Tripitaka was composed and edited by the Hinayanists several centuries after the Buddha’s life and expresses doctrinal developments that would be anachronistic in that era.
2. There is no particular reason to believe that Buddha and early Buddhism was against caste except in the general sense that they are against worldly behaviors of any type. You won’t find anything about it in the Aryan eightfold path (Aryashtangamarga) for instance. (Aryan is usually translated as “noble” in this case. Did you ever wonder why?). The ten major disciples of the Buddha were all either Brahmanas or his royal relatives. The first major historical Buddhist ruler Ashoka in his rock edicts exhorts his subjects to respect both “Brahmanas and Shramanas”. The Ashokavadana, the earliest Buddhist hagiography (and canonical in Theravada) states that a Buddha can only be born in a Brahmana or Kshatriya womb.
3. Buddhism is against karma and holds that liberation is from Jnana only. Animal sacrifices are mentioned a lot because they are particularly vivid examples of the negative consequences of karma but the Buddhist critique applies just as readily to lighting a ghee lamp for worldly reasons. The Jains were more consistently for non-violence than the Buddhists. (No predominantly Buddhist culture is predominantly vegetarian). Jnana only is the position of Advaita Vedanta too. The difference is Smarta Hinduism developed a modus vivendi where karma was acceptable for householders. There was no lay Buddhism during its Indian phase only the sangha so the question could be put aside.
4. Yes Astika philosophers and saints contended against Buddhist counterparts (and each other) but it is anachronistic to bring those disputes back to the time of the Buddha himself. By the time of Shankaracharya there were several mutually antagonistic Buddhist sects. (Actually three is generous. Traditionally there are said to be eighteen.). That is what he is referring to. They can’t all be “the true teaching of the Buddha” unless he was incoherent.