The simple answer is that he's parsed so much that it's engraved in his muscle memory.
I'm sorry, but that's not normal. No amount of muscle memory can produce results like that. The slightest lag or ability delay would throw it off by hundredths of a second.
Even looking at his previous parse currently on the leaderboard (the two of these being mysteriously in two different languages) it is clear that the ability is interrupted as fast as the system can possibly process it. There's a tick inbetween the cancellation and the loss of Unremitting, etc.
This 'player' cancels Master strike a single tick after the last hit, without exception. You cannot tell me with a straight face that this is legit.