I like to interpret that as the Asteromorph Gods demonstrating that with inhuman greatness comes a potential for inhumanly great spite.
It says subdued rather than killed.
The Qu considered themselves perfect in form and the universe was their canvas, and theirs alone divinely ordained for them to work on.
The Asteromorph would have played on that. The adult ones, the ones stupid enough to let themselves be taken alive, would have been put in old Gravital bodies but without the ability to change the world. They just get to observe for ever, never sleeping, never looking away, not even with eyelids. They would get to see as the Asteromorphs take their young and remake them into good little servants as they did to the Machine Humans, declawed, subservient and ever so eager and happy to please their new and wonderful masters.
The Old Qu would get to watch this, as their children are made into loving pets, debased from divine perfection and all their great works reduced to nothing so completely that it would be as if they never were, as if they themselves never were. They would get to know just how hard they done fucked up, how totally fucking insignificant they were before the glory of humanity and it's new extra-galactic friends. And they will get to watch this until the end of time, unable to look away or scream or die.