If you look at the letter as a whole, I think it is safe to say that he was alluding to the fact that Feynman had a great personality, unlike many renowned intellectuals.

To me, the quote from Wigner clears up the author's meaning. Wigner stated, "He is a second Dirac, only this time human." Dirac was brilliant, but he may have had a high functioning form of autism which left him socially crippled. He was painfully shy, he didn't have many friends, and social interactions with him were extremely awkward.