Actually you can find people that are really "smarter" that others, who get things noticeably quicker, even in research. Although as you note, being smarter isn't enough at university level.

But in any case they are a too small sample to make the modern science work, and most of the scientists are, well, average people maybe "smarter" than the average people but not by very much. What makes the difference is interest, motivation and opportunities.

And I say that as a former scientist that decided he wasn't smart (and interested) enough but who met many non-scientist smart people in his life