Some of us just went out and rode in the dirt and figured it out. Reading about how to ride in the dirt becomes more meaningful after you've tried things and noticed something that might be difficult to do. It's not like you'll get run over by a truck if you make a mistake in the dirt, you just dump the bike, pick it up, and keep riding. You occasionally have to replace broken levers, but that's part of the learning experience.
A lot of the enjoyment I've received from riding in the dirt was learning how to handle steep hills, sand, whoops, water crossings, and other challenging things. But I learned to ride on the street without official training as well and managed to survive. I don't think the MSF had been invented yet when I started riding.