It's not entirely clear. Obviously it's unsafe to operate an aircraft without sufficient crew. The full definition from the regulations is "required for safety purposes means required by law in order to reduce risk to passenger safety and includes required by safety decisions made within the authority of the pilot of the aircraft or any decision made in accordance with a safety management system as defined in subsection 101.01(1) of the Canadian Aviation Regulations but does not include scheduled maintenance in compliance with legal requirements."

I would argue that it's AC's responsibility to staff its operations appropriately, but throughout the pandemic, AC has taken the position - across its operations, not just with regards to APPR compensation - that nothing at all is within its control and therefore it never owes anyone anything. I imagine that, until a regulator or court tells them otherwise, they'll stick to that position.

I've moved your post here since this thread contains a few different APPR perspectives.