It might have a credible meaning, depending on how we explain the facts of time travel. Perhaps some agent inspires prophecies to help ensure a coherent timeline.

This suggests that HJPEV or Voldemort will, as a result of the prophecies, affect the distant past somehow and become their own metaphorical grandfather (e.g by 'erasing Atlantis'). But it doesn't have to mean that. Perhaps without post-prophecy Harry, Voldemort would simply have gotten bored with world domination and decided to kill his own grandfather again.