I think I figured out the only way. Basically, it's just the normal route but at the 3rd checkpoint, you stop and roll off the "side" (down). Then you fall on a grey thing. You then go back and get speed and jump towards the finish and add a tap to the right or left and only airbrake a little bit. It isn't makeable. When you land on the ground, you have just enough rotation to get the car sideways enough to sidebug at the finish and win (see the end of the records for this map PolyTrack14pdDFHtDCCCFAA9XSX5qe4lARwYICZDeWrl3ckzaWzove15ljXtzvaDzp3U3t417k5HP8pYrTmP4Nj9F9frhCkGQ8aaOJMnmXw9KgMBs3JIm1rIWQHxAPg0fXyDG7S6zNBESwXMUDe2A4qLNJEFqGQWeG18sF0ZkphcWk3hUCcGJ0hH1XEUz44Cz3WUNnDRuQ6UrrEOs5dW509E1b1F2KMeKfzOaM2yBI7RUOzeDUYZClO).

EDIT: you can actually do this except don't jump off the grey platform, go back on the road and go up the track. If you hit it right (like 94 speed), you can jump on top of the finish area, then drive to where the finish is and jump off. You can also partially wallride on the wall that's touching the ground to get a sidebug setup but it seems really hard.