An Ork can barely say that part of the sentence, much less THINK it.
I imagine Zayth as a pretty tough nut to crack. First, they don't seem to have easy targets; they don't seem to work together, generating supply lines to be cut, the people are firmly entrenched in their Big War, so morale is not really a weakness to exploit, and all of their cities, with all of their people, seem to be mobile fortresses with plenty of armor and firepower; they can even repel orbiting voidships, if they wish. Much like Cadia, I also imagine that most people there grow up, live, eat, drink, breathe, and die their war, so their population is also battle-tested, at least to a degree. After that, there isn't much there for most people to want. Your players, and the AdMech, might like some of their tech, and Orks just love to fight, so they get what they'd want, but no one else seems to care about the wasted dustland that is Zayth, outside its city walls.
If you could drop enough Orks in, I think they could give it a try, but they would probably lose, and without Nids to run amok, few others have the standing armies or the give-a-**** to try. My opinion, anyway.
Sort of a shame, too, because if a Rogue Trader COULD claim the planet, and the loyalty of its various tribe-states, it would be a great accomplishment, especially if, someday, a great threat to engulf multiple worlds were to finally strile the Expanse, such as a Hive Fleet, or Chaos Crusade; the people of Zayth, if not just ignored, will at that point learn that there are bigger, nastier things out there than their own little war, probably too late to save themselves.