A telling last line there Greg, which actually leads to the opposite conclusion, to the one I believe you wanted to make when you began your post. It may be that formal theatre - or at least just being an actor - was not the only way to acquire that knowledge and skill.
I am thinking back to what I read of Brecht's early career. He had already gathered some experience of singing his (offensive) ballads in pubs (and getting slung out for his pains) before he produced his first play, "Baal". He certainly knew a bit about theatre and he had plenty of in yer face performing experience.
Would you agree that busking, for instance, is a good cure for wimpy, introvert songwriters?