I wouldn’t go far as to say Von Neumann was smarter than Grothendieck. I think they’re both different types of geniuses, where their genius manifest in different ways. Grothendieck was a genius in working with extremely deep abstractions, I’d say he eclipses Von Neumann in this way, whereas Von Neumann had a different type of genius in which he eclipsed others at. In Grothendieck’s case he was a profound genius, who made profound impacts in mathematics.

Another mathematician that reminds me of Von Neumann is Euler. He also memorized long passages and could do complicated calculations in his head quickly.

A quote on Euler from wikipedia:

“He was able to, for example, repeat the Aeneid of Virgil from beginning to end without hesitation, and for every page in the edition he could indicate which line was the first and which was the last even decades after having read it”