They started working on art assets, sound and animations at the very least for this WIP trailer but even then the ingame trailer isnt complete (lots of reused assets). My guess is its all paperwork with maybe a few blockouts to prototype new gameplay ideas while the art team worked on the trailer

Have you seen the last Elder Scrolls trailer?

It would most likely have been a cinematic trailer, created by a VFX company hired for the job. ME5 was in early production, likely no game to speak of at this point, just storylines and concept art mostly. Maybe a vertical slice demo, but I doubt it.

Yeah and they repeatedly said it was in "early stages" of production.

From what everyone was able to cobble together with rumors and hearsay is that they had a core idea ready, but waited till after Veilguard to commit more resources to the project. I guess we can assume it got a bit further along before it was canned.

It looks like it was early production