This is all true, but — one of the core issues of right brain/left brain theory isn't so much about dominance as usability. Because the (nominally less "creative") left brain is the timekeeping and language brain, it's the fallback brain for most modern living, with its relentless need to organize, to hurry up, to get stuff done in the right order, faster, faster, faster, faster.
Learning to play guitar, or painting, or becoming a better writer, is about putting in the necessary right-brain hours of practice, yes — but for many people, it's a very difficult process to get the louder left brain to quiet down long enough to allow the practice. People who get frustrated learning to play guitar or writing or what have you will recognize this symptom — the idea of feeling like you're failing not in relation to the quality of the work or the practice, but because you're not getting better fast enough. That's the left brain talking over the right brain — not dominant, but just louder.