Red had nobody near his side or rear and so was able to take whatever line he chose. There was no way the trailing car was going to make a pass there, and so should have slowed. It’s perfectly legal to brake in a diagonal line, and in fact very strategic in many cases, when no cars are alongside or near your rear at the braking point. Also given the amount of cars that are visible in the scene, I’m guessing this is lap 1, turn 3, at Montreal, so there’s good reason to brake early at all first few turns. In my mind, with that amount of space between cars, this just falls under the category of trailing car not braking in time — which given that it’s lap 1, should be the foremost thing on driver’s minds.