This is actually a very smart play by the Masters winner (did anybody catch his name, by the way?)
If he had taken out Garchomp and Salamence that turn, he would have given two fresh Pokémon a free switch-in, with a weakened Conkeldurr and Hydreigon (who at -2 SpAttk can provide no real cover fire for Conkeldurr). His opponent could have taken a divide-and-conquer option, leaving Hydreigon a sitting duck while taking out Conkeldurr, turning the momentum in his favor.
Sure, with some prediction he might have been able to take out Scizor and Tyranitar with those two. But he has no idea what else his opponent has at this point. No idea if he can get his back two in without taking heavy damage. By sacking Hydreigon to get Specs Rotom in safely, he's playing it safe and making sure he can keep up the pressure.
Furthermore, by not going for the obvious Mach Punch he casts doubt that it carries it at all, and make his opponent think it's a Drain Punch\Wide Guard set.
By leaving Garchomp alive, he can execute a divide-and-conquer strategy of his own, because he knows Mach Punch will KO Garchomp before it gets a chance to move, so Rotom can focus on the other Pokémon safely. If he had taken out Garchomp the previous turn, both incoming Pokémon would have a chance to move and take out Rotom and\or Conkeldurr.
After having watched two of the videos you linked and cross-referencing the names between those semi-finals videos and the finals videos I found that the other two pokemon the person who got 2nd was using were Cresselia and Rotom-C. (Source
http://www.youtube.com/watch?v=BxL3q00TUJE and
http://www.youtube.com/watch?v=ZkAxJ8qyMpA)
Had he KOd both pokemon and he brought out Scizor you can check if it is Occa of Flying Gem Acrobatics with Flamethrower which you confirmed Hydreigon later had. Even Flamethrower and Drain Punch the next turn would have either put Tyranitar in the KO Range of Mach Punch again or Scizor would be incredibly weakened by Flamethrower in the KO range for Thundurus' Thunderbolt. Potential ways to win, Flamethrower with Hydreigon and later KO both with Conkeldurr and Thundurus or Conkeldurr Rotom-W or Mach Punch Tyranitar and switch in Rotom-W, had he used Flying Gem Acrobatics Rock Slide then Rotom-W takes the hit and can now threaten with OHKO on either pokemon with Hydro Pump, sure win.
If it was Cresselia Tyranitar in the back by using Drain Punch you allow him to send out Cresselia, you are either KOd or need to make a switch or Protect and if you none of them then you allowed Cresselia to KO Conkeldurr, this puts you in a disadvantage where it is now 2-2 against Cresselia Tyranitar and best move would probably be to Taunt an offensive Cresselia which can 3HKO Rotom-W with Psychic, and if for some reason he didn't Protect with Tyranitar the Tyranitar has over a 50% chance of surviving either by you missing or the damage roll. By using Drain Punch your odds of winning drop to about 20%.
Had you used Mach Punch in that situation Hydreigon will still be able to play and he can no longer use Psychic to KO Conkeldurr as then Hydreigon is immune and this makes beating Tyranitar much easier as you can Mach Punch once more to put it in the certain KO range. Worst case scenario you have an 80% chance of winning, this is four times better than using Drain Punch.
Tyranitar Rotom-C vs the remaining Pokemon, Hydreigon once again needs to be saved to handle max speed Choice Specs Rotom-C with Flamethrower. It is obviously choice specs as can be seen by how it OHKOd a Tyranitar (unless it was a critical hit, I can not read Japanese), in which case Thunderbolt will allow a sweep to occur once Hydreigon is KOd versus Mach Punch Flamethrower KO on the Rotom-C had Hydreigon remained.
This only leaves Cresselia Rotom-C remaining and as mentioned before both of these Pokemon would have had excellent match up on the remaining pokemon once Hydreigon was out of the way as there was nothing that can beat Cresselia in a reasonable time period, although you can time stall it will not work if you are down because you couldn't touch Rotom-C.
Scizor Rotom-C in the back would have created a loss for the other person in every situation though because even with Drain Punch you don't put anything in a specific KO range and offensively they will be able to sweep your team other than Thundurus which might be the only way to win if he choose to try to parafusion hax the Rotom-C because no matter what he did there would have been no chance for a victory.