Because to be a trader in the modern world you need an education. It does not even matter what type of trading he wants to do, be it global macro, quant or fundamental long/short. He is not going to get exposure to that knowledge on his own.
Additionally, if he wants to manage OPM (either as a PM or a founder), his chances are much better if he goes to a good school. I've been in the business for a little while and have yet to see someone who was able to make it on "the street smarts" alone.
Disagree if he wants to be a trader, you can also learn on your own.
For OPM generally you're right. But did you read his posts, he's already managing OPM without an education. So it's possible, he's doing it.