The reason lane "sharing" is legal is because there is nothing codified that defines how many vehicles can occupy the same lane at the same time. Thus, a car next to a motorcycle is the same thing as a motorcycle next to another motorcycle (assuming they are both within the same lane).
There is nothing saying that 1 lane sections make it illegal. It might not be the smartest thing to do, but it's not explicitly illegal.
As for the rider's actions in the video, it's hard to judge speed based on the video. It looked like some of that might have been too fast for conditions (in violation of 22350 VC), but I'm not exactly sure since I wasn't there to see it in person.