In fact, it turned out that Darth Vader was Lukes father.

Contextual fictional facts are really common in language. I remember doing it in philosophy first year. It conundrum-y philosophically, but usually understood effortlessly in normal discourse.

Well, you can be a descriptivist all you want, but it's commonly understood to not mean "in factuality" even if people use it "incorrectly."

In fact, Santa Claus is a man