Why does the rogue’s Greater Weapon Specialization specify legendary proficiency?

The rogue’s level 15 class feature specifies:

Your damage from weapon specialization increases to 4 with weapons and unarmed attacks in which you’re an expert, 6 if you’re a master, and 8 if you’re legendary.

How can a rogue get legendary proficiency in any weapon? As far as I’m aware, only the fighter reaches legendary proficiency with weaponry, but a fighter will never be able to qualify for this feature because it comes directly from the rogue’s list of class features rather than a rogue feat. Consequently, the two states:

  1. Has legendary proficiency
  2. Has access to this feature

seem mutually exclusive. Is there something I’ve missed?