Simply put, useful definitions of sentience can’t use mechanism for it to be a useful definition. First, a biologist can’t look at a human brain and tell if it is sentient, so it isn’t even useful to determine whether Fred my next door neighbor is sentient.
Second, we don’t usually have the ability to take something apart before we have to make a sentience call on it. For example, if I declared that only humans with type-A blood are actually sentient (they rest being mere simulacra), you’d still need to treat everybody as if.
Another way to put it is, if an alien probe of unknown construction arrives on Earth, how would we tell if it is sentient?
The answer, of course, is behavior. Until there’s a crisp definition that puts human behavior in one bucket and chatgpt in another, we have to consider the possibility that both are sentient.