It is intended that people not be able to obtain Superior Gollux pendants and belts without defeating the boss on its highest difficulty (or at least surviving while someone else does so....). Putting these items in the shop runs counter to that design.
Reboot in general has a higher equipment drop rate from bosses, precisely because one can't make up for bad drop luck by buying from others.. It's possible that this increased drop rate does not apply to Gollux, because it isn't KMS content. I will ask that it be looked into.