I can't view the clip-but assume the cyclist is holding him/herself up with a hand on a car.

I've only done that once, while on tour. Cycling into Istanbul proper I got really lazy in hectic/yet stop&go traffic, and reached out + held onto a utility trucks side view mirror-rather than unclipping. The folks in the truck were really cool about it, and didn't make a fuss. It took me a bit to realize that it wasn't that bright of an idea (brain to hand-"stay away from truck!"...).