Is there a term for this sort of fallacious argument? It's based on "either this person is a liar or what they're saying must be true". The third, not mentioned possibility, is that a person can fool themselves.

And if you're talking the scientific investigation of complex phenomena like language, fooling one's self is common. Indeed, unless a researcher is using tremendous care, their chance of at least partly fooling themselves is very high.

Scientific processes and checks and balances exist because of this. That Everett essentially ignored these and primarily argued his case in the press leads me being very dubious of him. But this still doesn't he's a "liar", that he believes what he claims is plausible but doesn't change anything else.