(prefacing this post with not all allistic people, and not all autistic people)
I feel like part of the reason you see so many autistic people who identify with non- or alter-humanity is because we have a fundamentally different understanding of things that society accepts as "just the way things are," especially if society made things that way in the first place and they're not any kind of universal truths or set-in-stone facts (think gender, social norms, conversation/communication rules).
Like with gender, we know that autistic people frequently have a different relationship with it than allistic people do. That's why there's so many autistic trans and gender non-conforming folks. Allistic people see boxes around that kind of thing that autistic people just don't, as much.
I think that's at least one big part of why there's so many autistic people who identify as non- or alter-human. Our brains don't always do that automatic "this is the way things are" thing that allistic brains do.
Allistic folks go "You're a human person, therefore your identity is contained within those terms," and a lot of times autistic people will go "I don't feel human in the way that I see other people experiencing human-ness. I feel a connection to something else, even with the knowledge that I physically am human. My identity cannot be contained in your terms in a way that fits comfortably for me."
Whether it's animals, concepts, objects, or whatever else, there are a lot of ways people can experience their identities that don't really mold very well to the shape of the box that allistics decided was meant to go there.