This, to me, is such bullshit. Not the part about learning a lot from teaching — I have no doubt that you do — but that you decide to teach a course because you want to learn a topic. Imagine the arrogance, the sheer size of the cojones to jump into a teaching role without having thoroughly internalized the subject that you intend to teach. It may be a great way for you to get into a subject or maybe even to assert yourself as having knowledge about a subject, but what does it say of your respect for your students?
I remember Kyle Simpson's course on service workers, which, as he admitted himself, he put together in order to have a chance to better learn service workers. I watched that course, where he was continuously stumbling over the syntax, and constantly referring to his notes — and then, soon after, I chanced upon a demonstration of service workers by a googler, who happens to work with them on a daily basis. The contrast between those two presentations was startling! What that googler demoed effortlessly over the course of an hour took Kyle five or more hours to laboriously wade through.
Experience matters, is what I want to say.