Howl confirms here that the poem isn’t just a poem: it’s the Witch’s curse, and he won’t be able to evade it for much longer. Notice that Howl seems to frame it as destiny, or at least as a guarantee, that he’ll have to go back to the Witch. But unlike Sophie, he’s going out of his way to plan how to avoid the curse as much as possible. He doesn’t accept it as inevitable and instead, as Sophie’s sisters did, is fighting to live life his own way.