[DoniandaCoado]

Access to Insight is wonderful, I agree, but the author here is uncritically repeating a sectarian myth. Theravadin, Sarvastivadin and Mahayana sources have differing (and equally self-serving) accounts of what exactly happened at the Second Council. Scholarly examination, though, has yielded the consensus that the Mahasanghika Vinaya was the more orthodox, with the Sthaviran version containing additions and deviations.

If you're interested, there's a discussion of the whole convoluted story in Andrew Skilton's "A Concise History of Buddhism," pages 47-49. Here's a link to the relevant section.

The problems posed by these contradictory accounts may be solved by a Mahasanghika text, preserved in Chinese translation, called the Sariputrapariprccha. It is the earliest surviving account of the schism to have been compiled, and it describes this Council in a rather different light. This text is concerned with a debate over matters of Vinaya, and explains that the schism resulted from the greater party (later to become the Mahasangha) refusing to accept the addition of rules to the Vinaya by the smaller party (later to become the Sthaviras)...

It could be objected that this is only another partisan account of the first Schism, but at least we can see that the Theravada and the Mahasangha are both agreed that there was a dispute concerning the observance of the Vinaya, the only basis technically possible for sanghabeda. Its story is supported by the fact that the Vinayas affiliated to the Sthaviras do contain more rules than the Mahasanghika Vinaya, and it is generally agreed that the Mahasanghika Vinaya is the oldest. Theravada as a distinct school got its start around the time of King Asoka, and it picked up state sponsorship. It seems there were purges of monks from other sects, and no doubt certain parts of early Buddhist history were retrofitted to make them favor Theravada's claims to greater authenticity.