Basically, at the end of the campaign, most of the Hives on Armageddon were still in imperial hands. The only Hive that fell was Tempestora (because of those ork submarines). And when the timeline stopped in 999 M41, the Imperium had gathered enough resources and forces to lay siege to ork-held Tempestora, containing the green-skins and preventing them from reinforcing other fronts.
The last reports on the situation around Tempestora stated that a strike force of three different
SM chapters was being assembled to retake the hive.
On the other hands, the orks controlled vast territories far from the fortified hives (the equatorial jungles and the plains of Anthrand for example).
They also still keep a massive fleet in the Armageddon system (although the imperials also have a massive fleet

), which allowed them to conquer the world of Chosin (the planet right next to Armageddon).