But I would point out that what she's describing isn't so much the principles of three separate "disciplines", but rather that there is a kind of triangular structure. On the vertical axis, you have "meaning"; horizontally, you have "reveal-occlude". Thus, decoration is at the bottom, art is meaning+occlusion, and design is meaning+revelation.
However, she starts to address why "bad design", which also occludes, is not art. She doesn't really do so, though. This is why: design is more than revealing meaning. As she implies, design also reveals purpose. Art does not occlude purpose: art doesn't have purpose. Thus, the reason why bad design is not art is because it has purpose without revealing it.
Lastly, I disagree with her litmus test of, "If they can't tell you why, they're a decorator." The test isn't whether or not they can communicate it. If they can communicate it, then why bother designing the product or creating the art? They're both communication media in themselves; if they could have just said it, what's the point? The test is whether or not they believe they're saying something.
Now, if they're trying to say something and failing, it's still art. It's still design. It might be crap, but it still counts as something higher than mere decoration. A pop culture shit-for-sound piece of music might be detestable and cringe-worthy, but it's still music. It's BAD music, but it's music, like your girlfriend singing in a horrible off-key pitch a song you hate, but she really loves. Music to your ears, man.
Of course, that's a discussion for another day...