Stasis doesn't stop time, it slows it down to an almost imperceptible level, but as I said 10,000 is a long time, even in stasis. If you go by Master of Mankind, even when he was hale and hearty, sitting on the throne proved taxing even for him.
The impossibility of Guilliman healing was that people were looking at it the wrong way. He was put in stasis to stop him dying from the wound, not to give it time to heal. Time keeps moving forward, so he was still dying, just much more slowly.
By that logic Guiliman should have crumbled as well like the Emperor.