I think though I may be wrong here (and am, @Catiette-style, thinking this through as I go) that there is something in the PP’s assertion that humans don’t have to be bipedal. Not “something” as in they are possibly correct, but “something” as in a thread that runs through this type of argument that can be pulled on to see where it goes.
I think it goes something like this: way back in the dark ages, people were bad and did mean things to people who were “different.” Enslaved them, put them in human zoos, abandoned them as babies outside of nunneries, stoned them, called them names, stopped them getting an education. Etc.
We are now enlightened, and we accept that though some people may be born looking or acting differently from the norm, everyone is human and deserving of [respect, a chance, an education, not to be spat on]. And, if person A doesn’t give [respect, a chance, etc] to person B because person B looks or act differently to person A, then the problem lies with person A being ill-informed about the world.
I can see that it would be very easy to go from there with not the slightest hiccough to “humans don’t have to be bipedal.” Because the assertion that the PP is making is about a person’s worth as a human, not about their biology. So, the argument goes this way around: “just because a person only has one leg, doesn’t make them less than human,” or, to put it more bluntly “one-legged people are human.” What they don’t (or won’t) realise is the argument doesn’t work the other way. You can’t say “humans are one-legged.” You can say “some humans are one-legged,” but the template for human biology is two arms, two legs (and the rest). (There’s a big hole in their refusal to “see” the argument from that direction, which is: how, if humans didn’t have a default template, would people ever have known in the past who to persecute? But I digress.)
If you only ever come from the argument from the one side, the “acceptance,” and “be kind” side, then statements like “humans have two legs,” and “only women have uteruses (uteri?)” must seem cruel and reductive and sure, bigoted. Because “I know someone with only one leg, and they’re still human,” or “I know someone who had a hysterectomy and they’re still a woman.” How very dare you imply that they’re not.
Such people are not processing statements about biology as statements of fact, because when they hear “human” they think it means “humanity” or “deserving of respect.”
And to be fair, lots of people in those way back when days came right out and said that certain groups of people were not really human, and were thus “less than” people who were really human - the ones with two legs, etc. And they definitely used biology to make these arguments.
But where the PP goes wrong is they assume that
a) we acknowledge that it is wrong to make value judgments based on someone not matching the biological template of what a default human should be, and therefore
b) there isn’t in fact, a biological template of what a default human should be (that people sometimes deviate from).
And they miss completely that there is no value judgement in stating that fully-functioning humans, as mammals, come in very specific forms.
They are, in fact, the ones who are in some way continuing to connect biology and value. Most biologists, and I would hazard a guess GC people, are not.