That would be one of those variables that makes it hard to be certain.
Worth noting though that if stat differences are significant, natures wouldn't affect much. To give a quick example:
Charmander has 65 base speed. Bulbasaur has 45 base speed.
At level 50, Charmander's Minimum w/ a hindering Nature is 63. Bulbasaur's minimum with a beneficial nature is 55, so Charmander still is faster.
It's true IVs and EVs could tip things, but I honestly don't think they put too much work into those factors, or traits. Given it's a demo, chances are they used some basic default value for simplicity of programming. Stats could be doctored, granted, to insure the given scenario in the match (it is a match, not a video; they're actually playing the game, though it's possibly choreographed to insure they use specific moves at certain moments.), but I think people are looking a tad too into this.
www.rpgdl.com The site Meeple argues at for too much of his life!