I would say, theory as in "music theory", not as "theory of general relativity".
A person who possesses music theory can explain why they are composing something they way they are, and suggest better notes to someone who is trying to embellish the music with a harmonization or whatever. Just like the people who held the "theory" of the compiler were able to spot better ways of implementing the ideas of the second group without spoiling the design.
https://pages.cs.wisc.edu/~remzi/Naur.pdf