I think you're right that it's more complicated than just assuming every hour of your life is worth whatever your day job pays you per hour. The point in the article was that what you do in your non-work time still has an affect on your work, and that's where the opportunity cost comes in. If mowing the lawn is stressing you out, then you're going to perform worse at your job and potentially impact your career in the long term. Or if you could instead be researching or practicing something that would make you a little bit better at your job, then you're missing out on potential future earnings as well.
Of course, the question then is how do you value that time? I don't think there's any objective answer to that, but it's probably more than zero.
EDIT: Of course, I just re-read your comment and see this is basically what you just said. I'll admit I got distracted and misunderstood your point the first time I read it.
Of course, the question then is how do you value that time? I don't think there's any objective answer to that, but it's probably more than zero.
EDIT: Of course, I just re-read your comment and see this is basically what you just said. I'll admit I got distracted and misunderstood your point the first time I read it.