The thread title is misleading, by the terms of the CBC report which he himself posted.
As the report makes clear, the reason the passengers were refused offloading was that Canadian Customs (CBSA?) officers were not available and would not permit the passengers to be offloaded.
This was an unfortunate situation and it is far from the first time that CBSA has been intransigent in this situation, but the fact is that YYR is a long-established diversion point, medical emergencies occur, and this could have occurred on any air carrier operating over the North Atlantic.
Unfortunate, but not appropriately either a UA issue or one for which UA is blameworthy.