That statistic is way off.
As I understand it, mortality is less than one percent and often reported as "nil." Of those who die, there are almost always co-morbidities like advanced age or fuliminant colitis.
It very well could be that this particular stat follows ileostomates with colon cancer. Without seeing the original document, I'd suspect that this GI practice is attempting to ensure its "continuity of care" among UC patients. Ahem.