They initially said that Abyssea might be accessible through Walk of Echoes, but it looks like now it's accessible through new cavernous maws. By the looks of it, they'll be in Konschtat Highlands, Tahrongi Canyon, And La Theine Plateau.

In this next update, it looks like we're getting more nation quests; I don't expect we'll get Walk of Echoes until the update after that.

Also note that the update requires both Wings of the Goddess and Rise of the Zilart for access.

That said, I think you're the one missing the point. This content appears to expand far beyond a single endgame event. I'm still not sure why exactly you're convinced it should have been part of Wings of the Goddess in the first place. CoP didn't have new jobs. WotG doesn't have new endgame. It's just how things go.