This is a well-known and important aspect of keeping data classified. Just because classified information is leaked does not mean that people with clearance can treat it as unclassified. There is lots of bogus "classified" information out there, and an excellent way to limit the harm of leaked truly classified info (which is unavoidable) is to make it indistinguishable from bogus info. And the way to do this is to not confirm or deny that it is legitimate.

Yes, this means people with clearance will sometimes have to sound silly when it becomes overwhelmingly clear that the info is legitimate. But the alternative is for individuals with clearance to effectively declassify information unilaterally, which would be disastrous. Instead, information is only declassified by folks at the top.

This is just the basic idea I gathered from working near labs doing classified research. Some of the explanations for seemingly bizarre behavior concerning classified information gets even deeper into the "because he'll think that she'll think that he'll think...." rabbit hole. Someone who actually has clearance can probably speak on this much more usefully than I can.