There's still a difference between holding back 90%, and holding back 99.99995%.

He was shooting to kill everytime against Boros and the other "true" monsters, and he walked into the fight with Garou fully intending to take him alive.

You can even see it in the scope of his power displays; he punches Boros full-on, but only does glancing blows to Garou's limbs, and really only uses Serious Series to get in close to him or rip up the terrain.

Til;dr: the fact that Saitama is always holding back doesn't change the bottom line: Garou surviving their fight is not a testament to his power, because Saitama did everything in his ability to make sure he did survive.