So I'm not super convinced in Dumoulin's shape but yesterday really wasn't ridden in a way that suits him.
Dumoulin isn't good at following fast accelerations, he never was. The way he rides big climbs is that he watches the climbers getting a gap, doesn't care, sets his own pace and usually catches the climbers sooner or later. When there is basically an uphill sprint he cannot do that though as there is no road left to reel in the climbers. Just look back at his results in the past.
He only finished 8th in the Gran Sasso mtf in the 2018 giro which came down to very late acceleration by Yates, which he couldn't follow. A few stages later Yates attacks right at the beginning of a two kilometre ramp, which on paper looks even worse for Dumoulin, gets a similar gap immediately but once Tom finds his rhythm the gap only got smaller.
The main reason why he won the Oropa stage in 2017 was because Quintana attacked early, got a gap and went too deep trying to maintain it. I'm pretty sure if Quintana attacks with 1 km to go that stage ends very differently. In fact, funnily enough, if Dumoulin wants to gain time on climbs he is basically relying on the climbers to go too early as that allows him to make the climb into a mountain time trial and that's where he can beat them.
What that means in terms of "should Dumoulin just commit to helping Roglic" is that I still don't think he should. Dumoulin is still in a position where everything is possible for him and as cycling is a sport where everything can happen any minute I still think having a 2nd card to play is the right decision for JV. What if Roglic crashes? What if he fades in the 3rd week (which, just saying, he has before), hell what if he leads by 5 minutes, catches covid on stage 20 and has to abandon. You never know and I feel like it's nonsense to give up on Dumoulin when there is no need for it. Where the f*ck is the benefit for Roglic if Dumoulin does the dirty work for him if Kuss could have done it as well.
Also, in case someone attacks Roglic when the group is already reduced Dumoulin will probably just ride tempo to close the gap anyway, cause that's how Dumoulin rides gt's.