This was a Critical Mass event, which is why the bicyclists are taking up all of the street as a way to reclaim the streets and protest the lack of safety for riders under usual conditions. It’s not legal, but protests are never useful if they’re fully legal now, are they.
Whenever I’ve been on a Critical Mass ride, we’ve always had designated “corkers”, people whose job it was to block traffic with blockading/dancing/whatever while the others continued onward. Without people doing that, you run the risk of this sort of carbrained nonsense.
Something else that happened on mine was that as soon as we hear sirens, we drop the mass protest and prioritize making a path for emergency vehicles to get through.