Manga/anime is art, it's just not yet considered Art. These are very different things. Art with a capital 'A' is the stuff that gets sold at auctions and goes into museums. Some manga art is considered Art (Tezuka's stuff for example) and is appreciated for its style and influence on later art. The value added to Art is largely false and easily swayed by fashion and sentiment.
As for art. Manga images (without text) can draw the same emotions, appreciation and so forth from a viewer, so given that, I feel it is art. It also has the same rigours (if not more) than an average anatomical sketch. There are many nuances to learn on top of what people need to know for realism.
It's funny really, because a while ago, one of my friends (who is exclusively a manga artist) was told by someone, "I get anime, and I kinda like it, but you're not a real artist because that's all you can draw".
She proved it wrong by topping various categories on that art forum - photo realism, animals, landscapes, portraiture, fantasy, abstract. Then went back to just doing manga. She has never taken any art classes, or learned to draw in the traditional style, nor does she even practice realism. She uses it for backgrounds.
So my point is, you can learn to draw well via any medium really, as long as you draw so much that your hand draws whatever your brain tells it to draw. The simple skills of observation, memorisation, imagination and putting those lines/pixels to paper/screen doesn't change across the disciplines. Even photography is the same, when you're composing a shot, you think of shadows/light/layout/line strength etc, but your brush is a lens.
When your hand knows how to follow your brain, that's when you have learned how to draw, and you can then let your imagination be your only limit.