Didn't he set the records for Pac-Man and Donkey Kong back in the 80s? If so, he couldn't have used MAME to set those scores, its first release was in 97
He set the Pac-Man score live at Fun Spot in NH. With Pac-Man, there is a defined perfect game. With DK, the game only ends when the memory crashes and guys have been coaxing more and more points out of the game for years.
In the (excellent)film The King of Kong, Mitchell is shown offering a score that was sent in on a tape and the documentary questions the authenticity of this score.