I never linked complexity to absolute certainty of something being sentient or not, only to pretty good likelihood. The complexity of any known calculation+experience machine (most animals, from insect above) is undeniably way more than that of any current Turing machine. Therefore it’s reasonable to assume that consciousness demands a lot of complexity, certainly much more than that of a current language model. To generate experience is fundamentally different than to generate only calculations. Yes, this is an opinion, not a fact. But so is your claim!
I know for a fact that at least one human is consciousness (myself) because I can experience it. That’s still the strongest reason to assume it, and it can’t be called into question as you did.
True, in a way. Without solving said mystery (of how an animal brain produces not only calculations but also experiences) you could theoretically create philosophical zombie uploads. But in this post what is really desired is to save all conscious beings from death and disease by uploading them, so to that effect (the most important one) it still looks impossible.
(I deleted my post because on hindsight it sounded a bit off topic.)