For example, everyone’s disks degenerate with old age. They squash, bulge, even herniate. But only some people experience notable pain from this. And back pain has been a growing epidemic for decades!

Are there physiological differences between people that explain this? Probably. But I suspect in many cases Sarno is right, it’s not purely physical. Look into neuromatrix theory and non-proprioceptive pain for a somewhat more scientific attempt at an explanation.