It’s a little unfortunate that Wynne doesn’t get a general description of the other Vinaya parallels to the introduction, though he apparently got some help with the Dharmaguptaka Vinaya. The Mahisasaka Vinaya has a very similar story, organized in the same way as the Theravada and Dharmaguptaka stories. And it also confirms that the hostility of the priest at Veranja must be a later Theravada addition. In the other two versions, Mara causes him to forget about his commitment to the Buddha, but otherwise he is well meaning and friendly. In the Mahisasaka version, he apologies at the end and makes amends.
So, Wynn’s right that in one branch of Buddhism - the Theravada-Dharmaguptaka-Mahisasaka branch - this was apparently an old story. However, it doesn’t appear in the other two that still exist - the Mahasamghika and Sarvastivada.
The Mahasamghika Vinaya sheds some light, though, on part the story. It doesn’t include the introduction about the Sangha being left without food in Veranja. It begins, instead, with Sariputra asking the Buddha to teach the Vinaya before it was time to do so. That part of the story is very closely replicated, independent of what comes before it in the Theravada Vinaya. So, it would appear that that part of the story is older than the framing narrative. The Mahasamghikas expanded on this core of the introduction in a different way: They tell us why Sariputra was so eager to have the Vinaya taught with a past life story about him doing the same thing when he was the minister advising a wheel-turning king.
After the past life story explaining Sariputra’s request, the Buddha travels to a place where the Buddha Kasyapa had once sat and smiles when he recognizes it. Sariputra then asks him to sit there too so it’d be a place where two Buddhas have sat. Etc. There’s similar stories in Pali suttas, I believe, if my foggy memory isn’t failing me at the moment. However, in this case, the occasion is when the Buddha tells Sariputra about ten benefits that teaching the Pratimoksa has. This concludes the introduction.
Neither of the Sarvastivada Vinayas include a similar introduction. The Mulasarvastivada Suttavibhanga begins only with a set of verses, then it moves on to the Sudinna story. Which is telling because that Vinaya preserves more myth and avadana tales than any of them. So, one would think an introduction like we see in the Theravada or Mahasamghika Vinaya would be there if it were not a sectarian creation.
So, then, I would suspect that the part of the story involving Sariputra is the oldest part and the story about Veranja before it was added later by one branch of Buddhism - perhaps Wynne is correct that it was sometime around the second council. The Mahasamghika introduction doesn’t have any material that could be connected to the second council, so it tends to disprove Wynne’s thesis that the Pratimoksa developed at that point. There was a major branch of Buddhism that didn’t connect those dots, overtly or covertly.
Further on, Wynne brings up differences between the Pali Majjhima Nikaya and it’s parallels in the Chinese Madhyama Agama. He doesn’t, however, seem conscious of the fact that MA is a Sarvastivada Agama. We’ve already seen that the Sarvastivadins didn’t seem that interested in establish the Pratimoksa as the root of the Vinaya the way other schools did. So, these differences are not quite as telling as he is making them out to be. What would a Dharmaguptaka or Mahasamghika Madhyama Agama parallel look like? We’re not likely to ever know, unfortunately. We’re stuck comparing Theravada to Sarvastivada passages when it comes to MA and SA, which is problematic when later sectarian divergences get in the way.
Another problem with that section is that he is only looking at specific parallels in Chinese sources to certain passages in Pali suttas. This is too narrow and doesn’t capture all the information that exists in those Chinese sources. I.e., just because the word Pratimoksa doesn’t occur in a couple sentences in a Chinese Agama doesn’t mean the pericope being investigated doesn’t occur elsewhere in the same Agama. But it does appear to make an argument against a thesis because these fine details usually do vary in specific passage parallels, and the actual occurrences that may exist in Chinese are ignored. This is the reason we have to translate these sources so that Indic scholars will stop doing this sort of thing and telling themselves they’ve made use of Chinese sources to bolster their arguments. Actually, they are engaging in confirmation bias by citing only parallels that support an argument, as far I can tell.
So, overall, I don’t think his argument is very strong that the Pratimoksa is a post second council development. The intro to the Theravada Vinaya may be, but I don’t think that says much about the origins of the Pratimoksa given what we see in other traditions. We really need more holistic studies of issues like this, but they are difficult because of language barriers and the academic dislike of translations.