When the ancient Greeks talked about other worlds, such as there are
other worlds besides our own, they did not mean world as in the planet
Earth, they meant the entire Sol system and the stars - essentially
another universe.

Most general books on extraterrestrial life that discuss early views on
the subject always mention the Greeks' discussion of other worlds with
life, but they don't explain what the old natural philosophers meant by
the word world, either because the authors themselves did not know
of the differentiation or they decided to ignore it so as not to blur the
issue plus to further support the idea of alien life being known and
accepted several thousands years back.

Kind of pophetic if you think about it in terms of our theories on
multiple universes today. Those Greek guys were smart.