What i feel happen is Roglič and his coach decided, to start relatively slow, due to the length of the time trial. Likely due to the fact, Roglič didn't prepare for this race, as a podium contender should. At minimum two months of preparations. The competition went in full gas. The rest is history.
It would be hard to win this year. Rohan Dennis had to prove something to himself and to the rest of the world. And he did manage to do that. As for Roglič staying close to Dennis. Likely that was just about gathering some useful data and experience for the future races. Roglič likes to test himself against the best. This year and on this race Rohan Dennis was the best.
Don't think it was a deliberate slow start. There's just a very big difference between one day ITTs and stage race ITTs, and different riders perform very differently in buildup, at peak, and after peak.
Especially in an ITT like this you have to be mentally ready to put yourself through a mountain of pain and Roglic probably didn't have the morale to do so until he saw Dennis flying by.
Things I don't understand:
Roglič has said multiple times after the ITT that he actually could hold the wattage he wanted to and that he executed the tactics he and his coach decided before the race. So he wasn't that disappointed after the ride.
This is where it gets complicated for me to understand. He was openly aiming for a medal before the event. Preferably gold because he already has silver. Did he really think those watts would bring him gold? How can he miscalculate it for 3 minutes? Maybe you can if you don't have enough experience. But here we are talking about a multiple ITT stage winner. He has won short, long, flat, hilly time trials in his career, four of them just this season. He beat Bevin, Craddock and Oliveira comfortably just a couple of weeks ago during the Vuelta. In Harrogate it was the other way around.
It's different in stage races, you say. I know. But shouldn't a ITT during the 2nd or 3rd week of a GT penalise even more GC riders who fight day in and day out to stay in front on every stage. Non GC riders (who aim for a TT) on the other end can just pick a spot or two where they show themselves otherwise they just try to preserve as much energy as possible to perform well in the ITT stage.
One explanation could be that the ITT specialists are that much better prepared for Worlds than they are for time trials during the rest of the season. This could be true but I don't think there's a big difference if any. Let's take a look at this year's Tour de Romandie. Campenaerts was in super form at the time. He'd just beaten the hour record a couple of weeks earlier. I think we can all agree that he is one of the best TT riders in the World right now, probably top 5. He would probably take the bronze without the misfortunes at the WC. In Romandie he and some other ITT specialists (Ganna, Kung, Martin, Bevin) were comfortably beaten by Roglič on a course that was similar to the one in Harrogate as far as altimetry goes (just 3 times shorter lol).
So the most logical explanation is that Roglič actually lied about the watts after the ITT Elite race. He was physically and especially mentally spent after the Vuelta. Only when he saw Dennis in front of him, he could push the way he was supposed to, but it was too late.