There's a very long expose on the entire Billy Mitchell cheating scandal that's been unfolding over the last several years by Veritas. Worth a watch if you're interested in how his cheating and usage of MAME was first uncovered.
There's no way in hell he wasn't using MAME. Now, it could be that the MAME version is an absolute perfect emulation that doesn't affect the final outcome. However, it'd be a whole lot easier to work some cheats into an emulator than the original hardware cabinet. If he can reproduce that score on a genuine cabinet, rock on. If he can't or won't, then I'd consider that damning.
MAME is a legitimate platform at TG, but it's not as simple as "play on MAME and submit just a video". At the time of Billy's scores, I believe that MAME play on TG required one of a few specific builds of WolfMAME, a fork that removes some options that would be useful for cheating. It also always requires attribution as MAME, as well as submission of an input log. Billy did neither of these things, so it doesn't matter if MAME is 100% accurate for DK outside of VRAM scanout. He submitted MAME runs as non-MAME runs, and thus did not submit an input log.