Or... maybe, just maybe...
wait for it
Two reasons. The first is that I've seen the Colossus kill the Repulse before Koth's monologue even finishes in normal gameplay, even in a single shot. That might have changed due to the new model, but still. Second, perhaps the Colossus doesn't fire on the Repulse because they're trying to get Koth to surrender, or they hope someone in Koth's crew will see reason. You know, "Don't do it, Koth! You're sacrificing ten thousand lives for nothing!"
But what do any of us know, really.