Of course "Ars Poetica" (Art of Poetry) is about art. What were you expecting it to be about? The life cycle of the mealworm? But it's also about how to experience art and, in some ways, life in general. For a moment we get to feel out of ourselves, no matter if the speaker is talking about poetry or birds flying in the sky. Any way you cut it, we get to go beyond the physical world and explore the metaphysical world of art and poetry.
Bottom line: poetry is about life. In MacLeish's poem, then, poetry must therefore resist defining something as indefinable as life.
Relax, everybody. Art and poetry shouldn't try so hard to prove something; rather, it should be just as free as life itself.