Hotter take: Freddy doesn’t actually know. The rest of the gang took those mortal forms to fuck with people but they were so endeared by earnest want to solve mysteries and help people that they’re along for the ride. And hey if Shaggy gets bodyslammed hard enough that it should have snapped his spine and he gets up fine after who notices? And if Velma knows things that she shouldn’t know on occasion, things that would have been impossible to notice without some kind of foresight or omniscience- hey, just a coincidence right? And if Daphne conveniently has a medley of strange skills- hey, rich kids are weird, right? It’s not too far fetched for her to have a hobbyist interest fencing, boxing, ballroom dancing, knitting, sewing, modern and antique fashions, and whatever else is relevant to the case at hand.