That's not true. They were clearly stronger than everyone else. If others would be dropped they could just fight it out. But I get what you are saying though. Basically Vingegaard was the first to take advantage of the situation, which were team orders demanded by Roglic, and Roglic was sad he didn't attack first.You cannot both be racing each other and not chasing each other.
At that point the situation was as I explained a few posts ago. Basically Rog thought there is no need to attack and just by following the others he would slowly regain the time to Kuss and win the Vuelta.
Alas Jonas did not care about the team and his teammates and so it happened.