Yes her kabal, which was a very minor one (and probably still is) lacked the numbers to take on a hive world, but she was still able to inflict massive amounts of damage on it by using her intellect and resources efficiently (tricking Orks into smashing into the world first). That it is fickle is irrelevent, it (and any number of other sneaky tactics) still enables a minor kabal to punch far above its weight and this is in addition to other kabals being known to sack entire hive worlds on their own.
So you have major kabals stripping hive worlds of their entire population AND you have minor kabals using their cunning to cause massive damage as well.
That example is breathtakingly terrible and completely flawed. Comet=random event, Dark Eldar using Orks=deliberate and calculated event. If the freighter was able to harness the comet somehow, then yes they would have fended of a DE attack.