I think once again it’s a lot more meta than anything. Mental gymnastics are required for Luke to be a legend in the galaxy - his story after Star Wars was very intimate and personal, not grand or anything. Lando, Han and Leia should rightfully become legends, but Luke? No one besides Leia and Han would even know he was on the Death Star anyway, the only people that saw him died…
Rian is playing up to Abrams’ fantasy of Luke being insanely powerful which was fed to him by over 30 years of Luke becoming the quintessential hero in pop culture - exactly because he’s not your common hero but much deeper than that, as Servii rightfully pointed - and of course the Legends stuff with Grand Master Luke Skywalker, single most powerful being ever to have been born.
Everyone expected Luke to be moving entire planets with his mind or something of the sort - he deserved it after going through such hardships in the OT. But life isn’t quite so good, and mistakes will haunt you until the end of time.
This right here is the problem - I’ve beaten this poor horse endlessly on these threads but to me the issue starts with RoTJ. Luke did not save the galaxy in RoTJ (beyond inadvertently preventing Palpatine’s escape from DSII). This sucks from the OT’s perspective because the whole ‘new hope’/‘only hope’ thing turned out to be BS - the Empire was defeated by basic war stuff. By extension the notion of the Galaxy (as expressed by TFA’s opening crawl) relying on Luke Skywalker to lead the charge against the First Order is immensely dumb. There is nothing in Luke’s actions (ie standing idly by while Palpatine destroys Rebel ships, refusing to fight Darth Vader, and finally throwing his weapon away for personal Zen reasons) that screams “yeah, let’s train a new order of pacifists to challenge this new threat”…