I think his thinking is That The fact that it can’t be locked up with full force indicates to him that it does not have the same stopping power as a disk.
I'm thinking your thinking and his thinking are correct regarding the drum brake not being as powerful—at least as it is currently configured, if not altogether.
That's
one of the reasons disc brakes supplanted drum brakes. It's likely he could lock up the rear drum brake if it were hydraulically actuated or perhaps if the brake lever were differently configured for greater leverage. Maybe the current system on his moto just needs more fine tuning; however, trying to demonstrate one of those other drum brake operation options might be an interesting project for a student to undertake.
Locking up the rear brake would seem to be mainly important to stunters, and bulked-up riders carrying full loads of touring gear in San Francisco waiting at stoplights to travel uphill. :giggles The ability of the rear brake to shed heat might be important to all riders. Front brakes seem to be universally considered as the best sources for braking power. I'm fairly certain riders were low-siding by skidding long before rear disc brakes became common.
Your son is attending motorcycle technician school and, if he hasn't already, should pose this question concerning the importance of lock-up capability in rear brakes—and why his drum brake won't lock-up—to his instructors. Opinions generated by that discussion should be shared.
