It's perverse to put people in a collaborative open plan office and make them be quiet. Quiet, focused employees do not need to be there; they can work from home or a closed office. And they should absolutely be empowered to do so.

This smells like a "hurry in and shut up" situation. While it's better to have quiet crammed mandatory office hours than loud ones, it's even better to keep people who need solitude out of the bullpen entirely.