Dude killed all monsters and did all secrets in juat unser a friggin hour, when i play it takes like 8 hours to do everything, and i've known and been playing Final Doom literally since it came out. Sure it's a TAS but it's art.
I assume he skipped killing the arch-vile on this level, it takes like 50 rockets to kill it so the rules allow it to be skipped. He does however kill the viles at the end of map 11 though, those aren't required either. It also took several years to discover it was possible to kill the vile iirc, so of course he's also missing 100% secrets on map 26 now.