Not sure what you mean. He had been working for 15km himself (with a few others), while McNulty, Martin etc, were doing no work at all in the first group, and still Evenepoel was one of the strongest on the steepest part of the climb.
He was over 3 km/h faster than McNulty in the ITT. That's a huge margin (over 1 W/kg ceteris parebus). Even chasing, even having better aerodynamics, even on a climb where it's easier to follow, he probably should have been able to drop McNulty.