This is definitely true.

In this case, you could go off of the fact that Blast was noticably concerned and surprised by what Garou was dishing out. Him copying his ability to open portals visibly shocked him, for example.

But I think there's a more important narrative reason as well: Cosmic Garou's main ability was copying others abilities, even those of Saitama, and he claimed to instantly perfect these abilities, making them even stronger. In essence, this makes him unbeatable, which is why it took a being with completely illogical power (Saitama) to defeat him.

I think Cosmic Garou was somewhat of a " Cheap Knockoff Saitama" with similarly unfair powers (this fits the narrative of Garou's prior internal monolouge about Saitama being the embodiment of unfairness). Unfortunately for him, Saitama's powers are just... more unfair.

As you said, it's impossible to tell who would have won, but to me, Cosmic Garou was a threat that, story-wise, needed to be defeated by Saitama and Saitama alone. If Blast were able to stop Garou in that moment, it would undercut the emotional impact of the following fight quite a bit.

In the end, it doesn't matter, though. It's just as silly and pointless as all the other powerscaling.