That kind of depends on one's definition of downloadable content/DLC, whether it includes "stuff that's locked away in game but can't be accessed during normal gameplay" along with "entirely new stuff that wasn't in the game files before"... Arguably, Game Freak has been promoting DLC right out of the gate with XY with that annoying "Blazikenite event-only" promotion... So I'm personally not inclined to take much stock in their currently-quoted view on "DLC", as I think the unavailability of the Blazikenite through normal gameplay essentially contradicts that stance. And even if one does take that stance into account, Game Freak can always change their minds at a later date/in the near future at the flip of a coin.
All data for DLC is not found in the original game. Its add on content through external sources that are compatible with the game and are read at the same time from the system's harddrive along with the game.
Blazikenite isn't DLC content. Its coded in the game already. Its a download gift that already existed in the game, but there is no in game location where the data will be read to receive the Mega Stone.
There have always been Pokemon and Item that were never found in the games and never have they ever been called or lumped together with DLC. The things we get from TPC and Gamefreak are pre-coded into our games, already exist and are not DLC.
I'm not sure how Nintendo 3DS games deal with DLC (as in, whether entirely new content not coded in the game at all can be added at a later date); the recent Lumiose City save glitch patch makes me think that it's possible. I'm heavily doubting that new Pokemon species can be introduced into XY (as people with the "entirely new Pokemon not coded in the game beforehand DLC" could cause incompatibilities with the games of people who don't have said DLC at all, unless the games do a "Griseous Orb"-esque thing like Platinum did with Diamond/Pearl where the older game is sent some sort of "patch" or something during wireless communication), unless Game Freak wants to risk incompatibilities. However, I believe that new DLC locations are entirely possible (I'm not sure what the location index numbers for XY have, but if they have an ambiguous "Distant Land" marker for Pokemon, then Pokemon from DLC-only areas can use that 'Distant Land' location in their Pokemon summaries and still be properly traded without causing issues with vanilla/original versions of XY), the same as Pokemon with atypical moves, items or Abilities that have been currently programmed into XY.
A Patch is not the samething as DLC and as far as I remember you could not use Sky Shaymin nor Origin Giratina when dealing with the older games. I know mine always reverted when I battled against someone with an older version of the games. Only time I remember there being any difference was with Hypnosis where the leader for the battle would be the one that dictated Hypnosis' Accuracy.
I can't remember if Grisous Orb would work when battling against someone with D/P or if it treated it as if Giratina didn't have an item.
I believe the same thing happens if you try to use the Kyurem forms when going against B/W. I don't remember anything being said about it but they don't exist in B/W's gameplay data at all. I don't think they can be used unless you face off with a person with those games with correct data. And the old games didn't have any Patches for them.
Patches also remotely fix error'd data in a game. They don't add anything new, just fix what's already there.
The three Pokemon we have now are already coded into the game so they exist in X and Y we just have to wait for their Data to be released by TPC.
Them saying they don't want and have no intention of doing DLC is pretty well cemented as a possibility.
Edit: As for how Nintedo does DLC, same as Sony and Microsoft. I have a few WiiU games that do DLC and it adds data that didn't exist in the game before hand into the game. Monster Hunter 3 U gives you new missions and weapons as well as a new Element to play with that would not be in the game if you did not do the DLC.
PMD's dlc isn't in the games either. The DLCs save to the 3DS itself and the game pulls data from the cart and the system at the same time to run DLCs. If you change 3DS' you lose the DLC play-ability for that game.
