ASRAAM
You make a very good point and I agree with you.
I expect that the rules were written in days when the whole braking system was not capable of so much retardation, the Grob has quite big brakes, modern tyres and was braking on a high friction surface, all these factors stacking up to make the certification requirements obsolete.
I suspect that with older types put in the same position the aircraft would have simply collided with the aircraft ahead as the ineffective braking system failed to retard the aircraft quickly enough.