This is the dungeon in question. And yes it is a dungeon; solo instances dont have treasure chests, and there is too much activity in the background for it to be an overworld FATE (which also dont have treasure chests now that i think about it). The reason I feel it is optional is because they don't show trusts fighting/running alongside us as in the other probable dungeons. Again, hopefully just the camera.
But if not, it might be to that Omega 2.0 enemy from the benchmark, what Akademia Anyder was to Archaeotania in ShB.