We literally didn't.

The information that existed 3000 years ago is more or less the same as it was, except ours is better, because we have tve concepts of fact and fiction, and we know the Trojan Horse was a mythical wooden horse in a real historical war.

If you watched Band of Brothers 1000 years from now. They will still know that WWII was an actual war and that Band of Brothers was a dramatisation that was produced decades later. The difference would be that you'd also have access to the imdb from which you can read it's history.

Just like the Odyssey was written years after the Trojan War. Back then myths and reality weren't as distinct as they are today. That's why we still tell kids stories about humanlike animals acting this way or that. It's not that it's "not real", just because humanlike animals are fictional, as it still teaches real life lessons.

Just like the Trojan Horse might be symbolic for the Greeks outwitting Trojans.

If our civilization falls, there's no guarantee that our common knowledge survives. It could very well be that people see a lightsaber and think that we had the technology to build one

Sure, yeah, people "see" a lightsaber... where? A toy? In the movie? So they've lost the understanding of what toys and movies are? I would really like to hear a short synopsis of the scenario in which you think this is plausible.