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.
Where I live, critical mass is legal. Bikes can go on the street and we have a law that allows multiple vehicles that move as one to be treated like one vehicle.
Where is it legal to drive in the oncoming lane?
Depends on where you are.
In most places you can certainly drive in the oncoming lane to overtake things in your own lane or avoid obstacles. You wouldn’t do it if something was coming the other way though, unless you really like the taste of airbag.
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.