What you're declaring as "the same" and "not the same" is completely arbitrary. Garchomp with Sand Veil and Garchomp without Sand Veil (assuming a second ability existed) is really as much of a choice / forme as Garchomp with or without Outrage. They are both options that the same "base Pokemon" have. How is an ability no less artificial than a move choice, EV, or item? All you have been doing is declaring that to be so.
No, it isn't.
Deoxys-A and Deoxys-S both have the exact same move pool, therefore they both have access to the same choice of moves. Banning Taunt on Deoxys-A but not Deoxys-S does not create a new alternate form because we are artificially creating differences between the Pokemon ourselves when they otherwise would have been the same entity. However, they are different forms because they have two entirely different base stat distributions, which when given a particular form, cannot be modified by the player whatsoever. I as a player can modify the base stat distribution by changing the form, but I cannot change, say, Doexy-S base stat distribution specifically. That is beyond my capabilities as a player in the cartridge.
With that in mind, Garchomp with Sand Veil and Garchomp without Sand Veil both have access to the exact same move pool. The only differentiation is they have two completely different abilities, which again cannot be excluded or modified separately in the same vein that Deoxys formes can't manipulate base stat distribution.
Choosing between abilities is akin to choosing between different forms, or rather, different stat distributions between the Deoxys, while sharing the same move pool.
Like base stat distributions, you cannot manipulate abilities (manipulating breeding to ensure which ability you obtained is no different than touching a specific rock in DPPt to ensure you have whatever form of Deoxys you want, and thus any stat distribution you want.)
You can however manipulate the move set. If I want to exclude the move Psychic from either Deoxys form, I have that ability as a player to choose that, provided a ban or rule does not prevent me from doing so. And it is this "player freedom" that ultimately leads to slipper slope bans and complex policies.
EDIT: Also, I would like to add to this, since I thought of another appropriate example.
To expand on the fact that banning moves and banning Pokemon with particular abilities are not the same thing, I'd also like to point to the different Rotom forms in Gen IV (when they still shared the same type.)
The only thing that separated these forms from each other (aside from the completely irrelevant sprite) was their exclusive moves: Overheat, Air Slash, Blizzard, Leaf Storm, Hydro Pump.
If we were to ban say, Overheat on Rotom-H and Hydro Pump on Rotom-W, they would not become the same entity, because we would be artificially imposing that similarity. The fact is that we have no way of changing the game code to prevent Rotom-H from learning Overheat, and vice versa; it's integrated in to their move pool, and cannot be altered. We can choose to not use the move, of course. But that is identical to choosing a different form, or even a different item or trait, which as I pointed out, is not the same thing.