Footage of the parking lot isn't exactly the well oiled machine I'd expect self driving cars to achieve. If they can't figure out a parking routine in a fully controlled lot how do they manage to navigate public spaces?
Leaving aside the likely-correct sibling comment that they don't own the land, double and triple parking is difficult for robots to do quickly / safely. It probably isn't a high priority and they might be more bottlenecked by getting vehicles in or out of the lot.
I think they react to each other the same as they react to human driven cars, ie there’s no programming of “that cars another Waymo so treat it differently” it seems like one of them sees “that car is about to back in to me” so it executes “friendly honk so they don’t hit me”. When maybe there should be a “it’s another robot, so just trust them” check in the logic, especially when it’s the middle of the night.