This is how I see it. The Sigils act as both a map and key. You nominally dont need a map to get to Zereth Mortis. You could learn where it is by learning the First Ones language and trying to find it from their artifacts/writing but that is extremely hard/would take too long.
Or you just use the Sigils to open a shortcut directly to Zereth Mortis. The Jailer could probably have done the former but even if he stumbled into Zereth Mortis without the Sigils he would not be able to do anything at the Sepulcher.