"Work for another company if you want to, but only for a small one..." My first job out of school was at IBM. This was invaluable not because I learned a lot but because I witnessed firsthand the bureaucratic inertia described in the article. When I left to launch a startup, I never looked back or second-guessed myself because I knew I wasn't missing anything.
Lastly, I think this sort of testing, if you will, is worth doing in general. There were a couple of things I was really curious about trying as a freelancer. Each time, even when it didn't pan out was invaluable in terms of getting to know my own motivations more and refining what I liked, was good at and what I didn't. Sometimes you think you love something but you really don't know until you do it. You might like only parts of it but its worth doing it to learn that.