"Why is plurality part of the alterhuman umbrella, no other neurodivergence is part of it"
This is why! I'm responding to a long dead discourse argument, but if you don't explicitly put your support for endogenic/tulpamancy systems out there, anti-endos will come and invade your communities. This is why I consider "alterhumanity should not include plurality" to be a fundamentally anti-endo argument.
Even outside of that, multiple/plural systems and otherkin have shared enough spaces to where it would be cruel not to include us. It's the same argument people like to use for trans people - "oh the LGB(T) community is for sexualities, trans people shouldn't be included" and while that may be true on a very literal sorting level, the communities have enough overlap to where if you said you denied one but not the other it wouldn't make sense.
Also plurality isn't a neurodivergence. Specific forms of plurality are, like DID/OSDD are neurodivergences, but plurality in particular is not a neurodivergence, because not everyone believes it is one. Saying that we shouldn't be included because it's a neurodivergence is like saying autism explains therianthropy, so they shouldn't be included in the alterhuman umbrella. Yeah, certain forms of therianthropy are autistic, but not everyone who is a therian will identify it as autism, a neurodivergence, or even as an aspiritual thing.
The only argument I've heard that has merit is the idea that it's lumping trauma survivors into a label that implies that they're less than human, but alterhumanity doesn't mean that - it just refers to ideas and communities that are contrary to the social definition of what it means to be human. Plenty of humans are in the community, and alterhumanity doesn't negate current humanity. Furries are also lumped into alterhumanity by virtue of people who wear an animal suit, and choose to portray themselves as nonhuman despite being human. Tulpamancers are also alterhuman, despite being humans (and tulpas not having a set form most of the time). Human identities are allowed in the community, and the idea that only nonhumans are allowed in the community is one the community has been trying to fight against.
Lastly, alterhumanity is an opt-in label (even though admittedly it's treated as opt-out, it is supposed to be opt-in.) If you don't want to consider yourself alterhuman, don't. There are plenty of plurals who do consider themselves alterhuman on the basis of their plurality alone, and you have no right to invalidate that.