While the fact that speech can silence sppech is true, the claim that asking questions of who has personhood “legitimizes dehumanization” is nonsense. In fact, everyone having to prove their personhood by using rational argumentation is an efficient way to silence irrational harassment and keep the market of ideas open! If more people took Donald Trump’s lack of rational arguments as a sign of him being subhuman, he would not have been able to mobilize an entire country to silence the market of ideas.
If you take any definition of who is “human” for granted without criticism, you effectively leave the definition to cultural whim. Consider the fact that there are no sudden genetic breaks between “species” in evolution, so who was the first human being? Since the present has no exceptional status in history, there is no qualitative difference between drawing an arbitrary line among those alive a quarter of a million of years ago or two million years ago, and those alive today. An emergent threshold in the brain makes much more sense, given that its exact whereabouts can be scientifically questioned to avoid arbitrary limits that can oppress. I did read “The greatest show on Earth” by Richard Dawkins for the first time when I was 17 years old and suffered under the dehumanization of legal guardianship, and I still shiver from the memory of his horrible claim that there was no objective threshold of adulthood but that it was legally “necessary” to draw an arbitrary line.
Imagine that Earth was invaded by aliens the intelligence of humans who claimed that it was “necessary” for the treatment of “mentally disabled” aliens to consider intelligence morally irrelevant and therefore treated humans as mere animals. Would you not protest against it? I would protest!