IMO, he only smiled because he didn't want to appear obnoxious, so he half pretended to be joking. But you know that deep down he believes it, and it's not such an outrageous claim. He's always been underrated, and he really is 'one of the best' of all time.
Maybe not one of the five or ten best, but top 50? Even the 50th best shooter of all time wouldn't be too shabby.