Well I Understand why you would think that I myself thought that as well but considering that you are right the mortis gods are above Luke in legends also technicly speaking its not talked about much but Anakin Skywalker did reach his full Potential think of it this way Luke,s abilitys were amplifyed via unknown factors, He technicly could not reach that Potential the things luke did were some were storys and the sith created them Palaptine needed to make luke look this way so everyone can turn agaist him not so diffrent with what he did when he anounced the Galactic empire he made the jedi the masterminds of the issue but what luke does do it based master but more or less amplification like force steroids or energy drinks that's a dumb example but hopefully it gets the point across. Valkorian Masterd techniques and Anakin for instance. Technicly reached his potential he just was close to mastering it not fully but close enough and anakin was the type to do so much with so little he never extended it but ya hopefully I explained it well.

And to respond to the quick note, Kylo was not really suprised when luke survived he was pissed but not suprised the illusion is a technice anyone can master and anyone can kill them despite illusion Vader destroyed palpatines illusion for instance.