There are people in this world who will never understand, say, the P?=NP problem no matter how much work they put into it. So to deny the above you'd have to say (along with Greg Egan) that there was some sort of threshold of intelligence akin to "Turing completeness" that only some of humanity were reached, but that once you reached it nothing was in principle beyond your comprehension. That doesn't seem impossible, but it's far from obvious.

"Does this mean that society would never discover certain facts had the most brilliant people not existed?"

Absolutely! If they or their equivalent had never existed in circumstances of the same suggestiveness. My favorite example of this uniqueness is the awesome imagination required first to "see" how stars appear when located behind a black hole -- the way they seem to congregate around the event horizon. Put another way: the imaginative power able to propose star deflections that needed a solar eclipse to prove.

I think a variety of things would have gone unsolved without smart people at the right place and time with the right expertise to solve tremendous problems like measuring the density of an object or learning construction, or how to create a sail that allows ships to sail into the wind.