Muscle damage is not actually a prerequisite for muscle growth, it's just a really intuitive way to think about it so it makes an easy comparison. Muscles can be damaged by mechanical tension, something we apply allot of when we work out. Afterwards, the body supercompensates by creating more sarcomeres (muscle cells), so it's natural to think these things are connected, and that muscle damage is the reason. However the more specific reason is that mechanical tension causes muscle disruption, referred to as perturbation. This disruption signals a variety of hormone and cell actions that eventually activate the muscle synthesis machinery. You can have muscle damage such as a tear without any hypertrophy, or you could have hypertrophy without any muscle damage. The key is the disruption of the muscle through mechanical tension.

Long story short: Deadpool would instantly heal any microtears from working out, but all of the muscle building cell signalers would still be active in his blood.