All true, but so far Mercedes has neither denied nor admitted the validity of the video. Looks like with as much doubt, they would have something to say about it. Of course, as long as they keep the mystery going, it's good advertisement for them, I guess.
Like I said, I ain't saying 100% that it didn't happen, just that by using the curvature of the tunnel for momentum, the physics of lateral force would reverse at the top of the tunnel and the tunnel would have to be curved the opposite direction on the downside to keep the car stuck to the surface. The curve compounds the momentum of force in one direction meaning the car would have to overcome that force to go on around. It would be more believable if the tunnel were straight all the way.
I may be wrong and this is just purely an opinion....YMMV.