Indeed - if anything his ego was _accurate_ - he correctly thought of himself as the smartest in the room. The question is, did his success lead to his ego, or did his ego lead to his success? I certainly do not mean to defame him - I practically worship Feynman - I've just been exploring the idea that a little bit of ego can really aid in accomplishing big tasks.
One of the earliest stories of his career was breaking into people's offices at Los Alamos and picking their safes to get access to their Manhattan project reports when he was impatient and didn't want to wait for them. I think he's had a healthy does of ego right from the beginning.