How did he do it?
Yeah I really don't know the details. Seems a bit like it has little plastic boards or something attached to his tires, then he uses rear paddle tires to push the bike forward. And maybe there are inflatable airbags inside the bike that help keep it on the surface. I'm assuming they did multiple takes to end up with this shot, but it's hard to say.
I actually read that the bike sank 30-40 times before they got the set up right. Crazy, no?