a lot of riders do quick little carves entering and/or exiting corners. i think they are like adjusting their line slightly. in my opinion it looks dope, and looks nothing like speed wobble.
Different pavements feel different during slides, some can keep your speed while sliding better than others but some pavement can slow you down more than others also.
I wouldn't think it would be that steep. A lot of freerider's that ride at that speeds find progressive hills. Like that hill just would of been really consistent in it's decent cause him to continuously pick up speeds.