I think the point is that a regular body weight routine is harder than using weights. If you're sedentary, doing a push-up is like starting out with bench presses that are more than half your bodyweight, finding an inclined surface to do push-ups against is not always possible, and using your knees can be painful, whereas if you're in a gym you can go bench press the 20kg bar or lighter dumbbells and know exactly how much weight you're using each session. Doing a chin-up on a tree in the park means you can't grab an assisted pull up band or use a machine to help you, so you're stuck trying to scramble up the tree with your feet + use bad sideways technique then lower yourself down slowly, which is risking injury.
These concerns are especially relevant for the elderly.