This only really works if a couple of other rumors prove true: 1) That your starter begins at level 5 with an elemental attack (probably Vine Whip, Bubble or Water Gun, and Ember) 2) The predicted starter evolutions of Fire/Psychic, Water/Fighting, and Grass/Dark prove true as well.
Reasoning: If 1 is true, the initial rival battle will be absurdly difficult if your rival chooses the starter strong to yours. A level 5 grass-type defending against a level 5 fire-type spamming Ember is practically an impossible fight. The first fight isn't necessarily supposed to be simple, but it wasn't supposed to be practically impossible, either.
With the predicted start evo typings (2), each fully-evolved starter is both weak AND strong to every other type. Therefore, the early advantage is eliminated when the starters evolve, balancing the rival fights, even if your rival chooses the Pokemon that your starter is super-effective against.
I think you're on to something, but if the rumors about the starters having an elemental attack to start are true. I think they would have the AI play really dumb in the first battle and only use Tackle/Growl and use their elemental attack once (maybe)