I always believed that Xizor was dead at the end of the book. That is why he never became a main villin in the EU.
On page 334, as Vader is watching his victory unfold, one of Xizor’s men is heard over the comms reporting, and right at the end while asking for orders, he goes “Highness?” because Xizor had stopped replying. Then the skyhook is destroyed.
I’ve always interpreted that as Xizor didn’t answer because he managed to get the F out of dodge when he saw the Executor bearing down.
So I’ve always felt Xizor got away, the fact that he wasn’t issuing orders before the skyhook was destroyed showing he had totally bailed before hand.