I don't know what a compositor does but I suspect that if someone were credited as a composer the average movie goer would think that the person was involved in the creation of the music. This is probably not what a compositor does so to call a compositor a composer would be misleading at best.

BTW, I ran this through the spell checker; it wanted to make compositor into compost and when I fumble-fingered goer to gpoer it wanted to make that grab.