Most people, including myself, think of DLC as 'Pay-For' content. As in, having to PAY FOR more pokemon, stones, and etcetera.
EVENTS and UPDATES, however, can add content that wasn't shipped with the cart, just like DLC. They've already proven that they can update the game, after all.
Nintendo and Game Freak probably knew that the carts would get hacked and reveal alot of stuff early. So there's a very good chance that additional content not held on the cartridge can and will be added - it's just not done yet.
Edit: Further ruminations:
Why have some stuff on the cart and not others?
Mega Latios/Latias, the faeries, and quite a bit of the other content may be unlocked through Gamestop/EB Games Distributions, or the pre-existing Internet Distributions, like Mega Blaziken.
However, it is possible for them to add larger events into the game via the EShop - new environments, for example, would be something they couldn't add through normal internet 'here's the pass for the Victini boat' distribution alone.
What could they add?
Though unlikely, free EShop distributions could hold anything from new pokemon to entirely new models for areas. It's irresponsible for them to have Pokemon hog so much memory card space with stuff like that, but then again, it is still an option.
Heck, with the Memory card to expand onto, they could theoretically expand the game as far as players Memory cards will allow - the most likely being 2GB, since that's what came with the 3DS.
It's not console or PC gaming drastic in terms of updates, but considering they don't HAVE to keep everything on the cart anymore, my guess would be that they aren't.