I'm not sure I follow your argument. It seems like you want to say, not that your uncle was wrong to claim that no one has any understanding of gravity because this claim is false, but rather that he was wrong to claim this because, in general, it is always wrong to claim that science doesn't know something. At this point, you seem to suggest that simply because it is always possible that someone has secretly or just recently explained a phenomena, that therefore, one is never justified in stating that a broad community of people lack such an explanation. On the face of it, this is simply an invalid argument.
Perhaps you are interpreting claims like "neuroscience has not yet explained subjective consciousness" as saying "Nobody on planet earth, as of the time of this utterance, has any understanding of subjective consciousness." Then, perhaps, you are claiming that no such person can make a claim like that, because they have not surveyed every living person. I don't think that claims like "neuroscience has not yet explained subjective consciousness" ought to be interpreted as referring to each and every thinker as of the moment of utterance - rather, they seem to be referring to the epistemic situation of a broad community of specialists. However, even if we take the above interpretation as correct, I don't think your argument holds water.
All of science procedes via inductive generalizations. We observe the behavior of an unimaginably small percentage of the electrons in the universe, and see that they have a certain charge, mass, etc., and then induce that all electrons have that charge, mass, etc. This is a justified conclusion. I would submit that surveying a large number of neuroscientists and discovering, not only that each of them have no understanding or explanation of subjective consciousness, but that each of them tells you that they have not heard of any neuroscientists who has an understanding or explanation, is sufficient to warrant the conclusion that no one has such an understanding. Of course this claim is defeasible, of course it could be false, but that does not mean that it is not justified.
It seems to me that your uncle had not even attempted to survey the scientific community (else he certainly would have discovered that he was incorrect). And it is for this reason that his presumptions were spurious, and not because one is always incorrect to claim that something is as-yet unexplained.