WvA looked so smooth on the descent, made me jealous of his ability to use the whole of the road when flying downhill.
That's the difference though: Van Aert was using most of the road but he wasn't drifting wide and he was half a metre from any apex. Alaphilippe was continually jinking away from the edges on the exit of the corners costing him time. It was interesting that Van Aert went ahead (easily) on the rest of the descent and Alaphilippe was happy to follow. He (Alaphilppe) was also much smoother through the corners after that.
I think it's down to technique/ability rather than tyre grip.
The sprint too: Alaphilippe's bike was going left-right which, presumably, was scrubbing the tyres and turning some of his power into heat. Van Aert's more linear motions were just propelling him forward.
It would be interesting to see their respective power outputs in the sprint: I suspect Van Aert's was lower (adjusted for weight if there was a gradient: aero wise they looked on a par to me) just because he wasn't "crabbing". If it had been raining, Van Aert would have been even better off; putting the power down when grip is reduced seems like a useful cyclocross techique
ETA: when we're descending, we can't (usually) use the whole of the road because they're not closed. Even if they were, we'd be nowhere near even the worst pro descender.