In my view, this isn't entirely accurate. On some occasions, musicians have invented theories for prescriptive use, for instance to overcome a creative roadblock. I think that modal jazz and 12-tone systems are examples.

Noting a neighboring comment, theory may not be necessary for composition, but some specific forms of composition may be prohibitively difficult without theory. I have musical colleague who composes multi part jazz harmonies for small to large ensembles. They are steeped in theory.

On the other hand, my spouse attended a seminar given by a fairly well known film composer. She asked him if he followed any particular school of theory, and he said no.