If fighting bugbears is the point, then that is what GNS theory is interested in, and that falls under its Gamist category.I feel you're evading the question. Forget I said anything about any atmosphere. That's not the point. Fighting bugbears etc is. In fact, if you wouldn't know that the GM is making it up on the spot, there is no difference to a preplanned dungeon delve. Any observer or even a player that wouldn't know that wouldn't experience things any differently. Still a sim just because GM is making it up on the spot rather than a day before the session?
You would not be alone in that. It's one of the larger historical criticisms of the GNS model. It does at least describe subcategories of simulationism, but it's a grudging nod at best. I think there was some kind of obsession in that whole realm with having things in threes....Also, I super feel that simulationism in this theory is just "everything else that didn't fit to our other categories we actually care about" category. Both narrativism, and according to you gamism have super specific and strict requirements to qualify, everything else is a sim.