If we find some Bigfoot like civilization living underground, but that's different to how we think of Bigfoot, we've "explained" Bigfoot and disproven "the wrong version" of Bigfoot.
If Thor lands on Earth and say, "I'm back," but says, "btw my name is "Bor". We've disproved "Thor". If we discover people with some disease which makes them live indoors, drink lymph fluid and live for hundreds of years, we've disproved "vampires" but explained the stories around that.
At the level of speculation we're operating at, these things can be argued both ways. It's not truth that brings you down on one side or another, it's your preference. You're blind to that.