The main basis of opportunity cost is knowing both outcomes of each decision and choosing the outcome that makes the most sense based on resource scarcity (ability to both strike and work at the same time in this case). While they may very well know how much money they'll lose by not working those 12 hours, they certainly don't know the true cost and gain of the same decision, thus making the opportunity cost unclear.

That's really the basis of my comment. It's not an assumption, it's quite black and white.