During the course of the race bits of rubber come off the tires and form what is basically a rubbery gravel. The racing line is cleared of it by the passage of the cars. Get down too low or up too high and you start sliding like you would on gravel. So he went high, accepted the slippery-ness and used the wall to keep him going the right direction as he kept the pedal down.

On the racing line, even with a clean track, they still need grip to turn -- he didn't.

Also to note that as it is the end of the race, they aren't on fresh tires and probably most/all of the field didn't have the full grip they would during other portions of the race.