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.
The point of a critical mass ride is to demonstrate that the cyclists are going to assert their right to the street whether car drivers like it or not, so drivers’ choices are to either support cycling infrastructure or stay stuck behind them forever. Not having cyclists use the road isn’t an option.
It doesn’t matter whether the drivers like the cyclists; the drivers must be made to realize that it’s in their own best interest to capitulate and demand bike infrastructure in order to get the cyclists out of their way.