Buying a little time is in itself a very minor feat compared to what Luke is known for. And why would someone have needed to help them? It's not like the Resistance hasn't been escaping the wrath of the First Order time and time again before.
"Buying a little time" is a massive reduction of the event. It saved the last remaining Resistance members, and involved an impossible feat. He basically went full-on-Jesus.
Someone needed to help them because they were trapped and helpless. The film clearly depicts this, and even has them crying out for help to the whole galaxy.
Regarding the First Order soldiers, in what way would they see their boss being humiliated? They probably crapped their pants when Luke survived the barrage, but then their boss chose to go down and face him alone. That's enough to be incredibly impressed with Kylo. And he didn't just face him, Luke vanished after Kylo put his lightsaber into him, so either Kylo defeated him or Luke fled. Even more cred to Kylo.
Luke punked him. Kylo took the bait and engaged in combat with a ghost. By the time he realized Luke wasn't there, the Resistance escaped and he failed his mission to wipe them out. The FO might chalk that up as a win, but they're still talking about it. The story got out.
How do you know that people talk about Elvis for no reason? I guess you feel that it's unlikely and if someone actually did see him that would just drown in the tons of rumors and false testimonies that are going around. Well, it would be the same for Luke. It would just be another rumor in the bunch, and created by some that are really desperate to get people to join their cause.
I know Elvis conspiracies are just that, because we live in the real world where celebrities die.
In the Star Wars universe however, Luke Skywalker is a legendary Jedi master. He is
the face of space wizardry, in a world where space wizardry is a real thing.
It should be clear that one of the above is much more likely to be discussed at large than the other. I mean, surely, right? People speculate about ordinary celebrity stories, how much would they talk about actual superheroes?
As for crazy gossip, I would again point to that the tiny Resistance actually blew up Starkiller Base. Something that could actually be seen. Apparently no one gave a crap about that so I don't see why they would get inspired by a rumor about the great Jedi Master getting off his ass to do one small favor to the Resistance, very late in the game, and then he won't help anymore. I'd say that great deeds that other people can actually follow are more inspiring than smaller deeds only space wizards could do.
The entire ST has massive problems with establishing the scope and scale of the First Order. Won't argue that. It appears that even without Starkiller Base they had enough resources to seize control of the galaxy. With no Republic fleet, there was no organized force to oppose them.
Consider the context too. Destroying that weapon wasn't as positive as the Death Star, where the plucky heroes managed to black the eye of the oppressive government. Starkiller was destroyed, but not before it was used to wipe out an entire system and a whole governing body with it. The First Order were a crazy fringe group with a horrible weapon. They were terrorists. Taking out the base was a good thing, but it was like stopping the shooter after the shooting. It still sucks, and there was still chaos and despair.
