Sareel Ja slipped into the Golden City portal and instead found Actual Factual Vana'diel. The existence of the Ark Angels confirms we're not dealing with a 'XIV equivalent', they're tied to a creation myth that can't possibly line up.
This is not unprecedented; XIV has previously used the Dissidia multiverse approach, putting other FF universes as off in the interdimensional rift; Gilgamesh came from there, Omega holed up there for a bit, and previously Lightning, Shantotto, Noctis and Iroha have crossed over, most of those unintentionally. It's really hard to successfully travel through the rift, so Sareel Ja definitely didn't do this intentionally, and there were equal chance that he could've come back with some really cool ideas for combat schools called 'Gardens'. (Well, in-universe; out of universe he was only ever gonna find the only other dimension with Mamool Ja.) EW Hildibrand shows us that you can get to a shard from the rift if you screw it up, so it stands to reason that a portal to a shard can do it too if sufficiently screwed.
I suspect the portal wasn't entirely functional at this point, as Sareel Ja had neither the key nor means to access it, so it probably dropped him in a position to see Vana'diel by freak accident, but I have no reason to doubt the story as it's given to us. (Layte_Aeon's theory that it coincided with the Everkeep's shift would track, too.)
...what I do have reason to doubt is Sareel Ja's own end of things; I have suspicions that we might have an 'unaware ghost' situation, and Sareel Ja actually did die when Zoraal Ja betrayed him and just hasn't noticed it. That would also be a plausible explanation for the portal weirding out, and could set him up for some weird final boss contexts; this fanbase does hate being given a villain they don't get to rough up.