Van Aert has been strong in the mountains as a domestique for a long time. This is pretty much the new normal for strong tempo-guys. Just look at the guys pacing early in the mountains for all the strong GC teams. How is it surprising that a strong guy with a good sprint wins from a breakaway ? It doesnt exactly make him a mountain goat does it ? His rival Van der Poel is pretty much the same type. Both can do well in a one off mountain effort, but cannot go hard day after day like the GC guys.