He didn't say that 4 years of university was a prerequisite to being a "free thinker", but I think it's probably useful. You get to live and work with fellow students who hail from sorts of backgrounds and have different goals and aspirations from you. Most universities offer classes in a wide range of subject areas, and many even require you to take classes outside of your specialization. There's a high concentration of intellectuals to interact with. And you get a lot of time to think about life, the universe, and everything.

There are certainly substitutes for all of those things outside university (albeit scattered about), and it's questionable whether a full four years if needed. But, all else equal, I think the liberal arts education is conducive to "free thought", if not necessarily economic earning power.