First of all, this is Nessie's characterization (distortion) of what revisionists argue. No revisionist has ever made such an argument (especially with "therefore").
Second, the word belief covers a broad range. A belief could be baseless and irrational. It could be self-serving. It could be based on careful, objective, intelligent consideration. It could be based on elaborate sophistry. In epistemology, sometimes you will hear the terms "justified" and "unjustified" belief. This recognizes that the truth may not be knowable with certainty (i.e., it is possible for justified beliefs to be false or for unjustified beliefs to be true).
Nessie is basically just doing the Big Lebowski meme.
You could dismiss any conclusion this way. This settles nothing at all.
"I do not believe hundreds of thousands were gassed at this camp"
The above is a statement of opinion. A conclusion. We cannot say it is false BECAUSE it is an opinion. That's just an assertion that we are wrong. The relevant follow-up question here would be: What are your reasons for not believing it? The strength of the position would depend on how strong the reasons are.
Now, could someone look at the same information and reach a conclusion different from that of the revisionists? Sure. This is called disagreement. Both sides could then give their competing reasoning and this could inspire further research, future books and articles, etc. But to fall back on "You are wrong because [circular reasoning]" is not very productive.
Reminders from the Forum Rules
Refrain from unsupported disagreement. This means that if you disagree with something or think it is incorrect, you must explain why you disagree or show why it's incorrect. A mere expression of disapproval is not sufficient.
Observe the principle of charity. "In philosophy and rhetoric, the principle of charity or charitable interpretation requires interpreting a speaker's statements in the most rational way possible and, in the case of any argument, considering its best, strongest possible interpretation. In its narrowest sense, the goal of this methodological principle is to avoid attributing irrationality, logical fallacies, or falsehoods to the others' statements, when a coherent, rational interpretation of the statements is available."