We know MacLeish was responding to Horace's "Ars Poetica", written in the first century A.D. So the title we get here isn't exactly original, kind of like the countless remakes and remixes of songs and movies we have nowadays. So the title very simply relates to us the "Art of Poetry" in all its elusiveness and mystery. And since it's a topic that's been contested and written about for so many years, MacLeish makes it a point to maintain Horace's original idea of poetry being "lasting," but he also keeps things fresh for us with those modern paradoxes and slant rhymes.
So, the title pays homage to Horace while also simply stating what the poem is "about." The irony of course is that MacLeish avoids telling us what a poem should be about. Instead, he lets us know that a poem should "be" rather than "mean." So the title that points to the general art of poetry sounds just as open-ended as the poem itself. We're not meant to cage the poem in any particular meaning. And likewise the title isn't limiting us to any one meaning.
So poetry is indeed an art form that, according to MacLeish's speaker, should be open and freed from the concrete world of meanings. And that's why poetry is an "art" and not a math equation. If art tried to define the world for us, leaving no room for adjustment or change, it wouldn't really be art. It'd be a math equation instead, which would make this poem's title translate more practically into "Art of Math." And call us crazy, but that doesn't sound quite right, does it?