The problem for Dennis is, that he never seems to do exceptionally well in TT's longer than 35/40km. (see also, his world hour record attempt). And the second problem is that the worlds course is really hilly, with even a 5km climb inside. It's not only the climb, the whole final 22km is up and down.
If Dumoulin is even close to his best, he'd beat Dennis on that kind of course.
Second problem, yes he wins a lot of big ITT because its his main focus. But he does benefit from Dumoulins higher focus on GC and lesser weight in many of those. The loss in the Giro prologue is actually very telling how close a rested Dumoulin would actually be, even in flat itt's