Chapters 43 and 45 don't seem to me to imply that baby-Harry actually heard the "strange word", only that for whatever reason Harry found himself thinking it. It was strange because he had (so far as he knew) no particular reason to be thinking that word.

I don't have any very convincing theory for why he had that word in his brain at that point, though. Evidently he interprets it as a message from his subconscious that he should think of Dementors as a riddle, or something like that, but probably something more is meant to be going on.