There's a lot of tacit knowledge in trades that was not documented, and in many ways can't be easily documented. You can't really document how to swim or play an instrument; you need to practice and train your body. Would you brilliant person from the 17th century be competent as a member of a tribe in South America or as part of a steppe clan in Asia?
Sure, I should have been more specific and limited the statement to academic or intellectual knowledge. Stuff like history, natural history, mathematics, astronomy, philosophy, major works of literature, etc. The point is that up until that time, a wealthy person who devoted themself to study could conceivably learn it all in a lifetime.
I've heard similar opinions before and considered them plausible in the past, mostly based on this idea that there were people who "invented" in multiple disciplines in a way that seems impossible today.
However, now I wonder if that might be more an effect of the increasing level of communication among humans in general at the time, printing presses, steamships, postal organizations, newsletters, etc, all combined to cause ideas to spread much more widely and more quickly than they did in the past. Perhaps its more a function of being the first (or most popular) person to synthesize certain ideas based on being at the intersection of spreading knowledge.
