Yeah so if Polar posts the battle, you'll see what I'm talking about, but this kid essentially just finished the Delta episode on ORAS and brought his freshly caught legendaries, without setup, to unranked Battle Spot Doubles. So to me, the unchanged Deoxys was a key indication that the player had taken minimal, if any, time to set up their team.
The people whom I've battled so far that run a threatening Deoxys usually run Deoxys-S or D. I'm not saying that the base forme isn't viable (hell, I'm the guy who recommended Dual Screens Raikou), but folks who like that mon just don't typically pick it.
Don't want you to think I was ignoring you, and you make a good point, Xerneas can speed creep my Dialga pretty easily. Here's what I do to handle that:
252+ SpA Life Orb Dialga Flash Cannon vs. +2 104 HP / 0 SpD Xerneas: 213-252 (50.8 - 60.1%) -- guaranteed 2HKO
Lugia gets a Tailwind off unless my opponents fakes me out or swarms me with multiple super effective moves, and even then, Lugia can usually take those hits. So that's a +2 speed boost for my entire team, then I come in with this:
252+ Atk Landorus-T Earthquake vs. 104 HP / 32 Def Xerneas: 196-232 (46.7 - 55.3%) -- 71.9% chance to 2HKO
I swapped my Choice Band out for a Choice Scarf, and now even a +2 Xerneas with max speed can't outspeed it. After Xerneas is out of the picture, Lugia and Mega Sableye can clean up shop.