The entire chapter wasn't committed to a single battle:
With regards to garrisons, one thing to note is that in M32 the Fists were still charged with manning (in a reduced capacity) the walls of the Imperial Palace, I doubt they would have random garrisons strewn across the galaxy as chapters in M41 are prone to have (they also would have had far fewer in general - a 1st or 2nd founding chapter would have almost 11,000 years of cumulative pacts and duties to fulfill at the end of M41).