Does your experience include LW rationalists deploying such a trick?

It's true that people will dishonestly move goalposts, but at the same time, certain claims really do require proportionally more evidence -- and the correct ones can produce that evidence (e.g. quantum "strangeness", evolutionary theory, etc.).

Such a level of evidence can reasonably be characterized as "ironclad" or "unmistakeable" -- and to borrow from EY's felicitous phrasing, it would take a heck of a lot of evidence to unmistake Mormonism.