Didn't only Warhol say that? Regardless, — Noble Dust

According to Google search and Wikiquote (and how could there be any error in that) they both said it. It could be that one of them was quoting the other. Or, it could be they both had the same insight, though the likelihood that they would have both conceived this insight in exactly the same words and word order passes unlikely.

It sounds more like Warhol than McLuhan, though McL was quite capable of the snappy quote. You decide for all time: Just say which one said "Art is whatever you can get away with" and that will be that.

Don't literary and plastic arts also require knowledge of what the given mediums, the forms, can and cannot do? — Noble Dust

Sure, but not nearly as much as music. Sculptors would need a great deal of material knowledge in order to take a chisel to marble or a spatula to clay. So, I know I couldn't write a decent short poem in iambic pentameter let alone a longer one with a formal rhyme scheme. Iambic tetrameter I can manage, and can manage a rhyme scheme too for short poems. (Iambic tetrameter is ideally suited to drivel poetry.)

Paint? I've seen some modern paintings where knowledge about anything didn't seem to be required, or even helpful. But knowledge about paint should have a shallower learning curve than the curve for writing string quartets or odd instrumental combinations (bagpipes and marimbas).

You decide for all time: Just say which one said "Art is whatever you can get away with" and that will be that. — Bitter Crank

Such power..

Sure, but not nearly as much as music. Sculptors would need a great deal of material knowledge in order to take a chisel to marble or a spatula to clay. So, I know I couldn't write a decent short poem in iambic pentameter let alone a longer one with a formal rhyme scheme. Iambic tetrameter I can manage, and can manage a rhyme scheme too for short poems. (Iambic tetrameter is ideally suited to drivel poetry.) — Bitter Crank

I don't want to get pedantic, but I'm still not seeing an argument for music requiring more knowledge of form. The only reason I care is that I tend to err on the side of the creative process being a fairly unitary phenomenon across disciplines. I think the process is the same, regardless of form, so one discipline shouldn't require greater or lesser control of form, just generally. I remember reading an essay by Henri Pointcare about the creative process of mathematical discovery, and his descriptions of doing work, failing, then coming back to the work after a day or two and occasionally having sudden, unexpected breakthroughs, resonated with me as well with my work in music. There seems to be this unconscious undercurrent in creative work, whether musical or mathematical, or whatever. People who do creative work seem to agree on what the process is like, on a general scale.

"unconscious undercurrent"

Creative activity seems to be mostly unconscious. Stuff flows out of one's head, without one having decided exactly what one would do in the studio, at the typewriter, with the marimba and bagpipe horror show.

bagpipe horror show. — Bitter Crank

I can think of plenty of things to do with that, just off the top of my head. I may or may not credit you.