Ivalice isn't the first time that major elements from another Final Fantasy game have featured in this one. The Crystal Tower was based off of FFIII. The Warring Triad came from FFVI. Certainly it's the case that Ivalice is more extensive than either of those examples, and that a lot of the storyline was imported into this game, but it's still just "FFXIV's version of Ivalice", not "FFXIV and FFXII take place in the same world".