Only partly true. This is true in the short term for younger people. It's not true in the long term for older people. Your metabolism results in different kinds of intracellular junk and intercellular junk accumulating. After a certain point, it reaches the level of pathology. But before that happens, using those metabolic pathways does tend to make them stronger.

Your liver grows stronger when you drink lots of alcohol, but that doesn't mean you're not killing it in the long term.

Same for muscles and joints. Consider somebody who works at a farm his entire life. Healthy when young. All joints break down at 45 because of the hard labor.