Of course, different professions have different amounts of "get a bigger piece of the pie" versus "make the pie bigger". The trades are more obviously positive-sum: if people get better at making houses, more houses end up getting built for people to enjoy. Things like patent trolling are negative-sum: when a patent-trolling office expands, they do so at the cost of putting others out of work at a greater rate.

However, this isn't the point that the "free college for everyone" crowd is aware of, let alone considering. It's straight equivocation between "college degree holder earns on average $X more" and "an extra college degree is worth $X more to society as a whole".