Missing the Point of Comparative Advantage

Daniel Hamermesh seems to miss the irony in his post on "Sticking to What I'm Good At" where he talks about volunteering two hours of his time basically weeding a sea plant (Freakonomics):

Productive groups generally learn quickly how to maximize output in situations like this, even with no guidance from a manager. Some people in the group (like me) were uniformly relatively good (or bad) at all tasks (had no obvious comparative advantage), so that their skills (or lack thereof) led them to spend the time alternating among all the tasks. I would think that primitive farming groups and, even further back, groups of hunters quickly learned who was relatively and absolutely good at which tasks.

Unless he gets true value in getting rid of sea grape, presuming that he makes more as a professor rather than a menial laborer (ditto for the opthalmic surgeon), it would seem somewhat more economically productive for them to just pay for the menial labor. It's sort of like people who supposedly go and volunteer to plant trees or dig ditches in developing countries - as if they didn't have much cheaper labor to do it with.

I'll side with one commenter here who says it's about the liberal guilt: "The point of doing something like this is to help rich people feel less guilty about vacationing in an island paradise."