His perspective in this is one that I can't really comprehend both from the lived reality of being plural and from being a cultural anthropologist, because of course there is no field of study which anthropology does not have its hands in. But basically if he was saying this stuff in like, an anthropology journal, there would be a thousand responses beating his ass into the ground within the following issue.
The problem with this perspective of his IMO, in what "Internet DID" could possibly ever be, is that there is functionally no difference between these two things ("Internet DID" and DSM-V outlines of DID). An individual is only ever as "mentally ill"/neurodivergent as they say they are and as a culture functionally finds them to be. That's not to say that there are no felt objective realities (I'm not about to become a subjective purist about this) but when you're working with someone who is trying to pull from the DSM as a strict guide, it doesn't feel like there are any analyses on the power structures that go into play here.
Who wrote the DSM and their motives, as you explain, has everything to do with what's considered symptomatic. That's why, like, schizophrenia ended up having a high diagnostic rate among Black people in the mid/late-20th century. These criteria are invested by institutions and the culture which enables them to have power and define norms.
tl;dr This is sooooo annoying of him and deeply unnecessary lol! (pissed)