History is full of very confident people denying the sentience of other humans simply by nature of gender, religion, skin color, age etc. Hell most people won't even admit non-human animals are sentient.
True, although LaMDA is neither a human nor a non-human animal, and it seems pretty clear that collectively we are far from sufficient understanding to evaluate what sentience might mean in such a context. Some of the standard biological tests for self-awareness (eg mirror test) clearly don't apply.