I read the misanthropy as ironic. They're applying the same reductionist logic to humans, not because they are misanthropic, but to illustrate that it doesn't help us understand the case we can all agree on. "Humans aren't sentient either" is definitely not the takeaway.
The point is, we have no idea what "sentient" or "intelligent" even means. If we agreed on the definitions, the debate would have been settled long ago.