They don't realize that for you to "Fake it," the thing you're faking has to be at least mildly believable in the first place.  Someone getting their PhD in biology and speaking with confidence about it when they're unsure of themselves, though still have the correct knowledge, is "faking it" to get past their Impostor's Syndrome. Someone who spouts absolute bullcrap while trying to claim they know anything about biology is just lying.