Lots of intensity there. I'm sure his lower back was fine. The stabilizer muscles were strengthened from squats and deadlifts I'm sure. That's all you need.
well I figured the squats involve the stabilizer muscles of your lower back and abs, they are isometrically worked, and then when you throw in the deads you do direct work through a full range of motion,
so the bent over BB rows would just work the area again, 1 arm DB rows would actually warm up that area and stretch it out in anticipation for the deads and squats