I thought it was strange that Flash think Garou was weakened by S-class. But, it made a bit sense. Because Flash didn't really see what happened to the actual fight.
Even he did fight against Garou and lose, Flash probably thought his speed just surprised him, 2 times. That is not enough for him to recognize anything.
Just like how Saitama catch Flash's attacks twice and Flash completely ignore all of that.
He is that kind of the guy who have to take many loses until he is sastisfy.