If I understand this point correctly, it means that if a rhyme {or a song} includes a referent to an actual event, we know that it couldn't have been composed before that event occurred. Is that what you meant?

Not exactly - though it depends on what is meant by "have been composed". A particular version of the song could not have existed prior to an acual event referred to in it, but that wouldn't rule out the reference being inserted in a song that already existed. And that is something that often happens, especially in playground rhymes, but also in a whole range of other orally transmitted songs.

It's analogous to what happens with jokes, where a contemporary figure will replace an older one. A Hitler joke becomes a Stalin joke, becomes a Saddam Hussein joke, and likely enough, dig around and it might turn out to have a prehistory as a Napoleon joke...