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.
You complain about one day of bikes being disorganized jerks in one place but you fail to reflect on how the automobile has utterly dominated the structure of all cities and urban areas. I think one is far more disruptive than the other and you’re focusing on the wrong.
That’s the thing, I actually agree with you and these cyclists. Cars DO take up way too much space and we’re way too dependent on them. I 100% support better urban development that’s more sustainable and pedestrian friendly. That being said, I don’t see things like this as a meaningful way to achieve results. If it was an organized protest that actually gave people a heads up that this would be happening then that’s fine, but just randomly forming up cyclist mobs to takeover streets is still dangerous and unnecessary.