It was a lot more linear though and a lot of the shots of story characters were pretty incidental - for example, the Scions are established but it also works as some shots of random prayin' dudes makin' the prayer magic work if you have no idea what's going on. Whereas the shots in the Heavensward trailer are quite specific. I mean yeah, you'd struggle to map End of an Era to the FFXIV story without playing it but it still works as a little standalone thing, it's cool and linear and easy to follow.
Showed the new trailer to somebody I know, he was able to guess, what was going on fairly accurately. I think you're really underestimating the average persons ability to reconstruct a plot line with only pieces of implied information.
Also, similarly, I've shown people End of an Era in the past, and they ended up confused by it.
"Who are these people, why are they fighting, why are they ignoring that moon that's about to fall on them."
End of an Era is excellent, but it's hardly a clear self contained story.