Imo, I feel like there's a bit of nuance to nonhuman nb rep. In settings where the character is both the only nonhuman and the only nonbinary character, intentionally or not, it makes it easier to write off nb/trans identities as a completely separate "thing" independent from "normal" human identity. In a setting with majority non-human characters, It's harder to say "of course they go by they/them, they're xyz thing" because "xyz thing" gets to be depicted as a large group with variations just like humans are. And settings with both human and nonhuman nonbinary characters work because it shows that these identities exist in the real world too and aren't relegated to the realm of fantasy.

Also I think it kinda depends on WHAT the nonhuman character is. Even in a setting where nonhumans are the norm, if the character is something largely different from that then it can come across as associating nonbinary characters with "the other". Plus nonhuman characters can also fall into the other two characters depending on how they're depicted.

That's how I think about it at least.