> We know that human brain cells in sufficiently large and complex assemblages are sentient

No, we know that human brain cells in specific configurations honed by billions of years of evolution and in specific states are sentient. (The same collection of cells in a slightly different state or a different arrangement might be permanently comatose).

This argument is no different in kind than the claim that Mamdani and El-Sayed are terrorists: it's a fallacy of affirmation of the consequent.

Sure (it was a mistake to say "sufficiently complex" without qualification, obviously only certain very specific complex assemblages will actually be conscious). But if you think the "honed by billions of years of evolution" thing is relevant (on which I agree with you), you also have to acknowledge that the same goes for the neurons themselves. And not just the neurons in isolation, but also their capacity for self-organizing into computationally useful structures, possibly in response to certain stimuli, which you can be sure this type of research will try to take advantage of.