[Punoxysm]

How can someone be a "secret" genius if they are one of the most famous mathematicians of the 20th century and received the field's highest honor?

I do not think that word means what you think it means.

[Kabukks]

I guess it's because he isn't as widely known to the public as other 'geniuses'. Or at least the people that label him as such feel that he isn't.