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You're comparing apples with oranges. Nibali is one of the best at 20-60 minute efforts in the high mountains, not 2 or 3 minute efforts on steep, cobbled surfaces rising no more than 150 metres above sea level.
This was his first Ronde, it was Terpstra's tenth. Knowing where to be on the approach to each climb takes years of experience so Nibali will almost certainly have wasted effort earlier in making up for poor positioning. I think it's astonishing that he was still at the sharp end in the final 30 kms, it shows what a talented racer he is.
Added: And you are right on QS' tactics, if they had pulled Terpstra back, one of the others would've gone.
BMC still has three in the bunch at that point no?
Maybe not to a Terpstra level but one of the should have been marking.