Hey! So I don't really know a whole lot about endogenic stuff, I mainly stay on the traumagenic side of plural discussion, but I wanted to bring this claim to you to see if it can be debunked since I've seen a lot of anti-endo folk use it
Claim: Plurality can only be formed by trauma because everybody is born with multiple parts, these parts hold functions like eating, sleeping, sadness, etc, but as we grow, these parts form into one identity. But when trauma happens these parts cannot form together and this create amnesiac barriers between said parts, soon once the person grows these locked up parts will form into multiple identities because of the amnesiac barriers between them.
I had a longer reply written up but Tumblr ate it.
Then again, this doesn't really need a long reply.
The model you're referring to is structural dissociation. This was a theory put forth to explain how trauma could cause DID and other dissociative disorders. But it's never presented as an explanation for all dissociation or plurality.
This model was first introduced and explained in The Haunted Self, written by Ellert R. S. Nijenhuis, Onno van der Hart and Kathy Steele in 2005. The first two of which also wrote this paper discussing their definition of dissociation that mentioned that there could be other ways "dissociative parts of the personality" containing self-consciousness could form.
There are lots of sources showing support from psychiatrists for endogenic plurality, but this one pretty solidly debunks the idea that all plurality involves trauma according to the theory of structural dissociation when even the theory's own creators are saying there may be other causes outside of trauma.