Look at the whole clue as a cryptic definition: Keats and Yeats seem like they ought to rhyme — same ending on the page and everything — but famously they don't (Keats sounds like "keets", Yeats like "yates"). So they're a rather ironic example of the answer. The cheeky "(or not)" is the setter giving you a nudge that these supposed rhymers don't actually rhyme at all.