Andy only says that to minimise tyre wear as North Weald is so abrasive.
Having heard it from the man himself as I asked why we do it - he told me it was mainly to stop the tyre from being able to roll under the rim as North Weald is so abrasive and grippy. With low profile tyres a lot of the guys run, you dont have to be sliding sideways much for them to grip and pop under the rim, completely buggering the wheel. Also it makes the tyre balloon which reduces grip meaning the cars will spin at lower speeds on the courses, which in turn means you dont have to go as fast to get used to the car sliding as it would on the road.
He told me the correct way to pressure your tyres is to look for the little triangles at the edge of the tread. If you are under it (ie wear is inside of the markings) you can lower pressure. If you are over them (ie wear is between them and the rim, actually using the sidewall) then you should increase pressure. A friend in his Evo didnt pump his tyres up as much as he should and actually we saw the wear on the tyre wall itself Increasing it a few more PSI made it go back behind the triangles and the car handled much better. Pumping it up to max would have meant the tyres had less grip due to ballooning, but would have protected the rims further and also not meant he'd have to go so quick to induce slide.