...Because it's neither of those things? Shit, you might as well say Loadout is just a Team Fortress 2 expansion, or every MMO post-WoW is just a WoW-expansion. The mobility system has a completely different feel, the maps are much larger and designed in different ways, the Mech/Troop push-and-pull is a unique addition to the formula, the campaign is designed in an entirely different way. It's a very different game that's a part of the same genre. Fez is not a Braid expansion pack.
If you're coming at this argument from a "Well I don't think this game is going to change the world" perspective, obviously you're going to be jaded. If you come at this from a "this game looks like it'll be a fun time" perspective, there seems to be a lot to look forward to with this game.