If you have decent fundamentals, I agree with this. Books are no fun; writing music is much fun. Do what is motivating. Copy others who DO know their fundamentals and beyond, and try to understand what they're doing as you transcribe and try to imitate their instruments/synths. You'll learn more from a "do first, ask questions later" method than from reading out of a book first without ever having formed any concept of what you're reading. Music is a little unique in that regard because it's so incredibly abstract.
If you play an instrument, do on the fly composition: improvise.
Improvisation took me from 0 musical ability to like... some musical ability, and it didn't take too long.