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.
Those cyclists? Many are drivers. This is a community event that builds solidarity and changes driver attitudes by mobilizing them against the vehicular violence we are all currently subject to. A critical mass ride has hundreds or even thousands of participants. It changes the way THEY think about cars, bikes, traffic, and public safety.