Nobody does that. We don’t think other humans are intelligent because of their behavior (just think about cases like locked-in syndrome or other inhibitions to ordinary communication), we think (and are justified in doing so) that other humans think because we do, and there’s no reason to believe we’re special. I grant other humans the courtesy of believing they have just as rich an inner life as I do way before, and independently of, the behavior I observe. Behavior may yield disconformatory evidence of course, but even there the inference is fallible (take again locked-in patients).
Nah. Just because a behavior is the product of thought in our case, doesn’t mean that it is in every case. I’ve used the flight of the bumblebee before as a metaphor: if we think bumblebees fly the same way airplanes do, we get the often-quoted tidbit that it should be impossible for them to take off. But the reality is simply that they achieve the same capacity through different means, so the inference from capacity to those means isn’t justified in general. Just as we have two models of flight in bumblebees and airplanes, we may well have two models of (say) language generation in LLMs and humans (and both empirical observation—the kind of errors they produce and the amount of learning necessary—and what we know about their functioning pull strongly in favor of that possibility).