As someone who is not completely in the loop on Billy Mitchell and the saga behind his cheating ways, what cheating did he do exactly?
Depending on the organization, game, and speed run category, there are certain requirements when submitting a speed run to the leader board.
An easy example is the category Billy ran for the Donkey Kong record, which required the run to take place on original hardware. Not only should the game ROM be original and running on authentic game boards, but the input devices must be original. This is so every record attempt is performed on an even playing field, where the only variable is luck and skill.
It has been proven beyond any doubt that a world record run he submitted was ran on MAME emulation, and not original hardware. He lied about it being on OG hardware and continues to lie about this. This run was recorded on video and the giveaway that he lied is in the actual world record gameplay video. Frame by frame analysis shows how the image of each frame was being drawn on the CRT, and the manner in which ithe games image was drawn is not consistent with any other OG arcade board ever analyized. It is perfectly consistent with the game being run on MAME emulation. Only one conclusion, he didn't play on OG hardware.
Another time he cheated was when he broke a record at some convention. There is a picture of him standing next to the machine, and the arcade unit he played on was modified to house a joystick that wasn't original and which allowed techniques to be used that or not possible on the OG arcade stick.
There is more of course, but I just wanted to give you some reasons why one can claim he is a cheater. Not to Stan for the guy, but Karl actually has some videos with a comprehensive catelog of all the evidence accumulated against Billy's claim of innocence.
Edit: I'm not an expert, and I apologize if I am not 100% accurate on the details. I'm relaying this info to the best that I can recall.