Did they even know about a Dash 8?
In the Linate disaster, everyone went for the MD83. They didn’t even know a Corpie was burning until a good while later. It was said had emergency services reached them in time, some in the Corpie would have survived.
In this case, you see a long streak of fire behind an A350. My thought if I was on scene would be that this whole flaming trail IS the A350.
Fire trucks responded to the Dash-8 because there wasn’t just fire on the ground, there was obvious structure/wreckage of some kind burning at the point of impact. Even if a two aircraft collision wasn’t immediately known (it may or may not have, based on what information the trucks were dispatched with by tower), but it would have become obvious pretty quick that there were two distinct wreckage scenes.
Skywest 5569 was thought to have departed before USAir 1493 even landed at LAX, so that one too wasn’t immediately known to be a ground collision.