The thing is, the best time to send Dumoulin up the road would have been Puy Mary, stage 13, when he was still only 3 minutes down on GC. The lumpy roads and false flat terrain in the last 40km would have been perfect for him, and if he had take with him one or two others who would not have ultimately represented a GC threat (eg, Porte, Lopez, Uran) and worked together, it would have forced a few other teams and riders (Ineos at the time, Bahrain, Movistar and, of course, Pogacar) into chasing.
Except nobody was to know for 100% until after that stage, and even moreso after the Grand Colombier, that Pogacar was the strongest threat, and that they needed to isolate him more that the rest. And after Puy Mary, Dumoulin was more than 4 minutes behind Pogacar, and represented a bigger threat to others.