I my interpretation of Their Finest Hour is the same with Scotty's. By destroying the Colossus, the Shivans have obtained their greatest victory over the GTVA - their most powerful ship and the symbol of the GTVA.
Their greatest victory over the GTVA in the second incursion was forcing the evacuation of Capella with the first Sathanas. The logistical strain of this would do far more damage to the GTVA's war effort than destroying the Colossus.
Or, if you prefer, their greatest victory was forcing the GTVA to adopt the plan
abandon Capella and collapse the nodes, destroying infrastructure work that goes back to since whenever the system was colonized (before the Great War, at the least). This also does vastly more damage to the GTVA's ability to make war than does destroying the Colossus.
Either way, killing the Colossus is nowhere near the greatest achievement of the Shivans.