It was 15 seconds, you really think the other teams we're worrying about Vermeesch and would've floored it if he dropped back?
At the point it was sub 20s the peloton had totally sat up anyway. When it was still 30s and Alpecin/Visma pulling, that's a big risk. They might have decided to drill it (independently of Vermeersch's presence), and then it's race over.
You have to think of it from UAE's perspectiv. A 30s gap would only likely close if the peloton sat up. Vermeersch is on good form and has come second before – if VdP got a mechanical, there was a genuine chance to win.
By the time it was sub 20s, the peloton had sat up. That wasn't going to be much energy saved for Pogacar, for what ended up being 2km of chasing.
Maybe UAE should have told Vermeersch to drop after the cobbled sector? But VdP had just attacked maybe the pace have would have jumped up and it was a lost cause anyway.