I'm not a classicist but I'm guessing you meant Homer? Though Plato used the reference to said horse I think he did it on the basis that his audience knew the story already.
Well if you were worried about a spoiler, perhaps you should have stopped reading when he mentioned the book name and that he had finished it. Or maybe immediately after that when he said "major plot point"
Did you know Darth Vader was Luke Skywalkers father?