The purpose of bodybuilding is to shape the muscles into their maximum size and shape. It doesn’t build strength to the same degree that powerlifting would build strength.
Makes sense I guess. Still, I should have just said weightlifting in general. What I think is, in a lot of strenuous jobs, you probably get a more natural range and variety of movements vs weightlifting, so that maybe these can result in just a different kind of muscle growth, or strengthening some muscles that are difficult to build with weightlifting.
I suspect most of Mantle’s powerlifting was hay bales or similar.