Silly comment of course your brake going off a slip road, totally depends on the slip road, I can think of loads where if you didn't brake you'd 100% crash.pity you can't see accident record for the audi driver, to see what other rear shunts he has had - as cheeseboy said,
you(anyone with experience/brain-cells) don't brake stupidly (if at all) going off a slip road,
presumably the intensity of the audi brake lights doesn't indicate if the car was deploying emergency assisted braking.
J12 A55, J3 M61 to make two I use regularly where you need to knock at least 30mph off your speed fairly quickly.