The Nebiros arrived at the last moment.
In fact, no other Shivan destroyer attacks the 3rd Fleet Headquarters. A valid proof.
How do you know they just wanted Capella to go nova? What makes you think that if left without oposition they wouldn't have ventured further into GTVA space?
They had two choices...
1) Make Capella go supernova;
2) Penetrate in GTVA space;
They sortied an high number of Sathanes, not even Luciferi. They wanted to destroy a star, not to bombard planets.
And how could they send in an immense fleet with Capella going supernova? They simply couldn't do both things.
Also, each time a Sathanas is sighted in GTVA space ingame, it had a destroyer close by. The first Sathanas had a Demon following it. The Sathanas that went after the Colossus had a Ravana going after the Colossus too just a few seconds before.
That is more a coincidence, or a gameplay choice.
didn't want Bearbaiting to finish after the departure of the Sathanas. And they didn't want to send Sathanas 17 against the Colossus. The fact that two Shivan destroyers preceed/follow a Sathanas is a coincidence.