100% true.
If you truly want to learn a language, first learn it in practice, then in theory. First learn how it's actually used on a day to day basis, be immersed in it, be bathed by it. Then later on, once you feel like you know it inside and out, that's when you start learning correct grammar. If you try and start learning a language through theory, you'll never learn to actually speak and understand it.
You know a good sign of when you've truly mastered a language? When you think in that language. Constantly.