Some people (honestly anyone, really) might believe that all word definitions are subjective. But they tend to behave inconsistently say, when someone insults them, when they receive a major diagnosis from an MD, or if someone straight-up lies to them.
It is this sort of inconsistent behavior that tends to contradict the belief that all word definitions are subjective. This contradiction becomes progressively clearer the more one interacts with such individuals. If someone insists on making up their own terms on the fly, then that individual simply cannot be trusted, and they're only tearing down their own reputation around themselves. See: Equivocation, doublespeak, and weasel-words.