Vaarsuvius started to address that; no one else in the Order was interested so it didn't get finished.
This remains a bit of speculation that makes no sense at all, with or without the reference to another book series that makes a point of not making sense. No one suggested that "one must believe they explored every single dungeon."If Xykon thinks that he actually visited doors marked by MitD, is it enough that the spell will also mark them as done?