> 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.