That's just a yardstick for how much it personally sucks (econ: "disutility") for you to go without that extra hour, estimated from your current pay. You could come up with a different value by deriving it a different way[1], but somehow you have to put a value on the time lost for purposes of comparison.
The existence of this opportunity cost doesn't require that you literally be able to put in an extra hour for extra pay.
[1] For example, you could go based on "how much would I pay for an extra hour to spend doing XYZ?"
The existence of this opportunity cost doesn't require that you literally be able to put in an extra hour for extra pay.
[1] For example, you could go based on "how much would I pay for an extra hour to spend doing XYZ?"