It's also a willpower thing. He never missed a single day, never touched the AC, and also fought monsters on the side.
It's a gag, but also basically how characters limit break. Monsters are what happens when you fail the will save, broken heroes make it and keep making it. It's the difference between Watchdog and Phoenix Man.
Garou's "training regimen" was getting the absolute fucking shit torn out of him over and over, logically that's worse than Saitama's mediocre exercise.
He stayed human the entire time and just willed himself to look like some shitty Demon thing.