Caedmon wrote a hymn and it's called "Caedmon's Hymn." Sarah writes a hymn and it's called "Sarah's Hymn." So far, so straightforward, right? But only if Sarah also happens to be the author of the first surviving whole poem in English. When you're old and famous, titles are simple.
What's important here is that Caedmon is mentioned—after all, it's not every day that an illiterate cowherd is transformed miraculously into an eloquent poet—and that his poem is given a specific form. "Hymn" covers all the bases: it's a simple, religious song full of praise for God. And by "song" we mean exactly that. This baby was probably sung rather than recited, with a harp strummed in the background.