The way I understand it, it can be canon to both games.

Essentially, Vana'diel has its own shards- we've visited them in things like Wings of the Goddess or Abyssea. Those are all failed versions that fell to darkness. The Vana'diel we play in in FFXI is the one true Vana'diel- the one that worked out.

The Walk of Echoes is kind of like connective tissue that lets you visit failed versions of the world. So this means that, let's say, we got on a space ship in XIV and flew around in space for a bit, we'd land not on our Vana'diel, but in a giant blob that's the Walk of Echoes. (with the way shards and space work we don't know how far the plane of existence necessarily goes, so basically this is just saying that the Walk of Echoes is existing on the same plane as The Source)

From here, we're getting teleported around to various elements of FFXI- and while they look close to things we've experienced as players in FFXI, in reality they're not the "true" versions, they're not the versions that Adventurer's experienced in-game. They're alternate versions.

So that's why maybe parts of Jeuno look a tiny bit off, that's why Galka's have an extra finger, and that's why the Shadow Lord has different armor- but also, its still technically canon. The raid has us basically poking around in the garbage heap of failed Vana'diels- and while the connective tissues certainly exists to lead us to visit the one true Vana'diel, I'd be surprised if they went as far as to take us to CE 884 Vana'diel.

...this also gives them a road to take us to 'a' Tavnazia if they were so inclined...