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 organizers will inform the city that a critical mass protest is taking place, the city will inform the police and they will cordon off the route. If there are enough people talking part in the protest, the public order authorities will have no other choice - thus the name critical mass. The right to protest takes precedence over the right to drive this specific route.
that’s a different setup than the one OpenPassageways described.