JM trained as a powerlifter at the worlds strongest gym for 10 years..... You cant pull that out of context, he started doing that as a 40+ year old master IFBB.
I'm pretty sure he's been a bodybuilder since he was 16, no?
Anyway, on the topic of JM and ham curls I liked doing something he recommended to help feel hams in compound movements and warm up the knees.
Do leg curls starting low weight and work your way up until you cant hit 15 reps. Each rep pause at the top, squeeze and then take three seconds to lower the weight. Repeat.