I'm pretty sure he didn't cheat until way after that point.
He still claims to have been awarded the title "video game player of the century" by Nintendo, when the supposed plaque does not say that.
He made an entire movie about beating a high-score, King of Kong, one high-score which is now in question. [EDIT] I have been corrected, he did not make the movie, he only played a part.
I'd say the high-scores are at the heart of Billy Mitchell's financial success and personality, which is why he is defending himself so vehemently against all criticism.
Even if the MAME proof is pretty good, he always manages to muddy the waters enough that people prefer to settle instead of rolling the dice.
I think if reviewed by a jury that understands the technical explanation the MAME proof is pretty solid, but nobody has been steadfast enough to take it all the way though an expensive court case and a trial. Twin Galaxies not doing that to settle it once and for all has been a massive disservice to the community. (That they settled in a way that explicitly does not call out Billy for cheating, but also not reinstating him fully, is such a cheap cop out is hurts)
They should have folded right away if they were never interested in actually proving the cheating in court. Did Billy cheat multiple times in collaboration with at least one other proven cheat? I believe the MAME proof can be interpreted that way, but since no jury has ever found such, it is still up for debate and will likely remain so forever.
[EDIT] For those not in the loop, the "MAME proof" shows how different emulation software and the original arcade hardware load the levels and point out that in videos of Billy's highscores the level loading does not look like the original hardware loading, but displays distinct patterns that supposedly only happen in MAME emulation, thereby proving Billy did not achieve those highscores on original hardware. Using emulation software you can, in theory, load gamestates and pretty much create save-files from which you can resume play if you make a mistake.