usage correctness and fact correctness
U have sorted this out very nicely, Nicholas W, by distinguishing between usage correctness and fact correctness.

But, can one "debunk" something which, on the weight of all credible evidence, is self-evidently true, namely, the fact of the Holocaust? I think not. The fact that one is sincere in arguing a case which is patently false does not make the truth "bunk".

Where there is no "bunk", there can be no "debunker".

On the other hand, where the proposition which is challenged is less than patently false, one can claim to "debunk" it by marshalling credible evidence in support of that challenge. In this situation, an etymologist cannot say that the word "debunked" has been misused even if he/she is not persuaded by the challenger's evidence.

Perhaps "debunking the Moon Landing" falls into this category. I really don't know. I haven't taken the time to examine the so-called evidence and I have never questioned the historical record. One would have to go deep into the woodwork to make a case either way. And the witnesses are few, and the scene of their deeds beyond reach ... quite unlike the situation with the Holocaust.