There are such things as second jobs, side jobs, and piecemeal work...
Cannot agree with this. I'm looking at it from the flipside of the coin.
Watching TV instead of brewing still has the same opportunity cost - you could be making money. Interpreting outside the economic lines, the opportunity cost is enjoying additional time with your family, cooking a finer meal, or getting more exercise.
Just because you accept an opportunity cost does not mean that subsequent decisions can't account for it at a later time. The analogue would be buying a $50
gift card, then walking into the store 6 months later with it and picking out $50 items, then considering them "free". They obviously weren't - you have just already accepted the cost and shifted the cost in time.
In other words, saying "there's zero opportunity cost because I would have watched TV instead," doesn't mean there's zero opportunity cost -- it just shifts it from the decision you would have made to watch TV instead of x, to the decision that you did make to brew instead of watch TV, or brew instead of x.