Your teacher is referring to the ekphrastic nature of W.H. Auden's poem "The Shield of Achilles." An ekphrastic poem is one in which the poet writes about a piece of artwork, such as a sculpture, painting, or another literary work that inspired them.
In this case, Auden chose to write a re-imagining or interpretation of Homer's Iliad: specifically book 18, lines 478–608 of that epic. So, what are the humanities that your teacher is referring to? I think you could potentially make two different cases here. First, you could consider the fact that this is a poem that is written about a piece of visual art (the shield) first described in the Iliad. Because of this, it incorporates two distinct categories of the humanities: poetry and the visual art/metalworking techniques represented by the shield.
Another approach you could take would be to argue that there are two literary genres present in this work: poetry and Greek mythology. It can be argued that Greek mythology is its own branch within the humanities. Since the telling of Greek mythology is through the written word—usually in the style of an epic poem—it is a sub-category in literature.
In the case of Auden's poem, he is using one humanities field (poetry) to recount a story within the Greek mythology canon, which is a different humanities field.