It doesn't even matter if he fully knew what was going on. Fact is, Garou and PS had a long ass fight from their perspective where they made god knows how many attacks, dodges, leaps and counters. Saitama just smashed Garou to the point where he couldn't really react to any blow and only broke out after it occurred.
Garou here had already powered up to the point where he could blitz Platinum Sperm (who performed the feat), increased his powers against Saitama, transformed twice (explicitly increasing his abilities a lot) while simultaneously increasing his powers with evolution, received a massive amp from God, boosted his abilities further by copying Saitama, and continued to evolve even more.
Even if he was just a bit surprised by the speed of what was happening, that's probably more than enough for the Garou that fought on par with PS, let alone Garou here.