To clarify, the door itself can be opened after simply getting 100 points for the art gallery and listening to the Ly story, but doing so has no effect on the story until FE2 is completed, upon which you can actually get invited to Lance's house before clearing FE5, but you eventually reach a point where you have to clear it, so yes, I essentially worded the guide in such a way where it means: "Don't open this until beating FE2 and FE5." You won't break anything by doing it earlier, but this way ensures a smooth flow.