This species is also very territorial, so any creatures that are vaiguly ape shaped, like humans, would be seen as interlopers and attacked.
Meaning anyone that does happen to get to close to the valley just disappears in the wilderness, never to be heard from again. Between, winter, wolves, bear, moose, and accidents there are enough dangers that a small number of trappers/explorers could go missing from bigfoot attack without anyone noticing.