So say that I'm a student. I take his advice at face value, I find a startup worth looking at, I hop in to help out for no pay. Maybe I do it in my free time, mid-semester; maybe I do it during the summer break. Whatever. I'm enlightened enough to know that I'm working for the education. The business doesn't advertise for it; it's a personal decision that I make to work for them.
Cut to John Doe. John is the son of Mr Doe, who owns a law firm. He spends time off working at his dad's law firm, because he intends to become a lawyer. John is not paid.
If I am told to cease and decease by a lawyer/my school/a policeman - no deal, I'll just stop and say: gee, I was in it because I wanted to learn stuff. Out of my own free will. I can't? No problem - I'll just stop. Ditto for John (though for some reason, I doubt anybody'd actually punish John, or even Mr Doe).
I don't see how I'd get in trouble. His article is directed at the non-technical student, not the employer. (If it was written for an employer/startup founder, then perhaps you're right to suggest that he should disclose that he's giving law-breaking advice. I'd be fully behind you on that one, qeorge, I would.)
But the article's fine as it is. And there's a corollary, of course. Entrepreneurs are relentlessly resourceful. If you're not relentlessly resourceful in figuring out a way to get into a startup for the experience, then you're probably not going to make it anyway.