I think the point being made is ultimately the same for all here. The thing about "psychology" is that it's a mind game. Nothing more or less, really.
Physically, You can lift what you can lift and you can push as hard as you can push. Push yourself too hard and you'll die, hurt yourself, or literally be unable to continue for a time. That's a literal thing.
But no one's mind is 100% (I'm betting) in sync with that. In our minds, we can all push significantly less than we actually can (be it in sheer strength or overall effort). Some are much closer than others, clearly. But we're all on the road of improvement in this regard.
Bringing our minds in sync with our bodies is the point. Not living up to ones physical potential is a mental thing.
However, allowing that we are only as strong mentally as we are, instead of expecting perfection in this regard both from ourselves and others, needs to be balanced against excuses and invalid justifications.