Sorry, you're missing my point entirely. I can only ask you to read it again. None of the following is a modification of what I said above.
I chose the Moon Landings as something in epistemologically the same situation as the Holocaust, but without the emotional charge, i.e. it is impossible to reasonably doubt them.
But someone certainly can claim that they are bunk. There is no semantic problem with this at all. The person so claiming is factually wrong, and unreasonable, but if they think it's bunk, then the expression 'That is bunk' is a (usage-)correct and reasonable expression of their views when they utter it.
And if I say 'Irving thinks that is bunk' then I am also describing the facts correctly and reasonably.
(Mental verbs like 'think' or 'believe' don't preserve truth across them. If Alice believes Oslo is in Denmark, and Bob says/thinks that Alice believes that, it doesn't entail that Bob says/thinks Oslo is in Denmark.)
Someone can claim to have debunked anything whatever. They have not actually debunked it unless they convincingly establish that it's bunk. But even if they don't establish it, they can make an attempt and claim to have succeeded. This claim might or might not be reasonable, but being reasonable is not inherent in the notion of claiming. I can (and frequently do) claim to be the King of Patagonia.
You can claim to debunk something no matter what your evidence is. You do not succeed in debunking it unless the evidence is compelling.
