Consider a "model factory" where 60 % of employees are union members. A strike is initiated and those 60% go on strike. It is reasonable to expect that production decreases with about 60%. In the PostNord case, the union member relationship is about on that level, and the registration plates probably fit in a small van. Still PostNord cannot see a way to meet their obligations even though there are plenty of employees that can deliver. If a non-union employee refuses, then their strike strike is wild and illegal.
I think the courts will see this, but it will take a while.