Surely the aircraft would still be moving forward relative to the air,
Yes. However, the motion through the air is not always purely horizontal, and aircraft can have motion through the air which has a vertical component. At the bottom, pilot pulls, aircraft pitches up, though motion downward continues (with the addition of a stall warning horn now) and impact. I've erred into this a couple of times. If the surface is not firm, it'll grab you, and stop you in a short distance (I have experience with that last summer).
When I train pilots on draggy aircraft like amphibious floatplanes, this scenario is a factor in the training, and is practiced. In the same aircraft, a glide approach could be at 60 knots, or at 80 knots. The 80 knots glide will give the pilot time to flare, and arrest the rate of descent, the 60 knot approach in the same aircraft will not. If the surface is not firm, it's going to grab you in any case.
I'm not saying that this is what happened in this accident, though when I see an upright aircraft (which is good) on the ground, with the engine peeled off, that comes to mind.