The trouble is that there is a very easily exploitable ambiguity there and so the phrase winds up being a motte and bailey for assholes.
The word "political" has connotations, and one of those connotations is a degree of intention. A political cartoon is a cartoon that exists to convince you of something or blast someone the cartoonist doesn't like, not "a cartoon that happens to be about an election". A political essay is an essay that exists to convince you of something, not an abstract meditation on power, and so on.
So when someone says "all art is political", whether they mean to or not, they are dragging in that connotation. They are implying intent, because it's part of most of the common usages of that word. Unless you immediately clarify, and/or are writing for an audience of humanities scholars, people are going to see the connotation and take that as part of your meaning.
... And assholes, particularly in queer and/or radical communities, love to exploit this ambiguity. They can tell you "your art is regressive/bougie/-phobic trash" without ever directly saying so, in a way that looks plausibly nice and neutral to people who aren't familiar with these dynamics.
A pretty common example: "young transmasc artist, shouldn't you stop writing about your experiences and center women instead? Writing about men is a political choice, after all." (And its evil twin, "young transfem artist, shouldn't you stop writing about your experience and center cis butches/cis femmes instead? Writing stereotypical femme women/trans butches is a political choice, after all.")
... It sounds neutral enough, unless you are running in these circles regularly and hearing the things these people say about the "wrong" political choice on the regular. I'm not going to repeat it; suffice it to say it's not nice. If you're spending time with people like this, you eventually learn to read "all art is political" as a threat. And there's a depressing number of these clowns. I think most artists on Tumblr have been on the receiving end of this bullshit; the ones that aren't currently in a cult aren't going to take it kindly.
..... The concept is a valuable one, and we shouldn't throw it out just because shitty little bullies like to misuse leftist language. But if we're going to have productive conversations, you've gotta remove that ambiguity. "All art exists in a political context" is a much better way to convey the same idea, because it's harder to suggest that the artist is somehow being maliciously political at you.
