Not really in a place where I can listen to the video for details, but I guess it depends on how easy that is to set up. Is that one of S.Mewtwo's BnB attacks anyways (haven't played him at all)? If so that could be tricky if he manages to get the phase shift and close in, but then you could choose to counter it I guess? Also looking from the vid, half of the round is over and normal mewtwo still wasn't at 1hp while Shadow takes much more damage, so Shadow needs to make sure it has the time to bring the opponent low enough that it could cheese out with probably...Espeon?
This will certainly make the field phase very important against Shadow, as losing it early could trap you. It's definitely silly, but there does appear to be conditions to it:
-Win the field phase to make closing in easier (difficulty in doing this varies by opponent, obviously)
-Correctly guess that opponent will block on wake up rather than counter
-Have enough time to lower opponent HP to point where you can cheese out the support/burst heal because you're losing HP WAY faster than they are for any lead to matter since you have the lowest default HP and you're losing 20 for their 18.
Those are my thoughts from just seeing the infinite. If Shofu mentions something in that video that invalidates everything I just said...my bad, can't hear the vid right now.