Anyone watching or recording would have seen:
-The pair briefly sparring, Kylo not being able to land a hit on him.
-Kylo charging at an unarmed Luke with a swing that should Luke clean in two, except it doesn't.
-Kylo sinking his blade into Luke's chest, and he doesn't flinch.
-Luke disappearing after some parting words, and Kylo screaming in rage and defeat.
It would have gotten out. I simply don't share this gripe with the film.
And Luke trying to punk Ben was yet another blow to his character. Not even at that point could he be bothered to try to get through to his nephew and old student and try to help him, like he should have done in the past. He truly was made to be the worst Jedi.
Don't equate my sassy language with Luke's intentions though. It wasn't a petty prank, it was a diversion. Luke told Leia he wasn't there to save Kylo, but he went out there and spoke to him, admitted he failed etc. Luke also assured Leia that "no-one is ever really gone" so it appears he saw hope, just not in
him saving Kylo, which is understandable and almost certainly true considering the circumstances.
Space wizards die as well. That's why the Jedi Order is gone. The only ones that have seen anything of some Force users living on in some form are other Force users.
That's not really the point though. The point is about why this story would be better believed than any other speculation and rumor about Luke. The notion that he suddenly shows up to help the Resistance after all this time, in a place where no one else could notice it, and then conveniently vanishes again, sounds like the typical story that's made to be unverifiable so people can't just prove it wrong,
That such a story, from a tiny group that have all the reason to lie to get someone on their side, is inspiring the entire galaxy, but no one cares about them blowing up Starkiller Base, is laughable.
I think I adequately demonstrated how word would likely spread within the First Order itself. In that case you could call it pretty verified if the enemy is leaking it. If you don't agree then I'm not too inclined to drone on about it.
It's not that no-one cares about SKB, it's that its destruction happened after the Republic was wiped out and during a galactic hostile take-over, so the victory isn't quite a savored. Still a victory though. It's not an either/or situation.
There were organized forces in every system as canon has established that the Republic didn't have a centralized military, everyone just had their own, so they were organized and controlled by local leadership. That means that the First Order has to deal with systems individually, which would take a very long time since a galaxy is extremely large, to say the least.
I'm not knowledgeable of the EU so I'm just going by the movies here, but what I saw in TFA was a Republic Fleet being destroyed along with the planets. I don't know of the size of each planet's local troops, but apparently not many organized and armed enough to assist with SKB or come to the Resistance's aid in TLJ.
Their initial plan of showing incredible force and then have everyone bow down in fear of suffering the same consequences was a very logical one. Nothing even remotely close can be done when you lose that power though. It's absurd that they control the galaxy within a day, when they likely couldn't even communicate with each system in a day's time. The one realistic achievement they got from blowing up the Hosnian system, and losing Starkiller Base afterwards, was just that the Republic couldn't launch a coordinated assault on them within any reasonable time.
Destroying the Death Star wasn't even near as big, seeing how the Empire was already more than strong enough to have had an iron grip on the galaxy for decades. Systems couldn't just rise up when inspired by that because the Empire were already present there with their forces and infrastructure to keep them down. The First Order never showed any capability of controlling an entire galaxy, so they were absolutely dependent on Starkiller Base.
I find it to be on the level of if ROS had opened with saying that since the Resistance escaped Crait they now have amassed a fleet to match the First Order in the next day. Unfortunately that pretty much happened as well, just later in the film
Again I am not inclined to disagree that the First Order were vague and badly drawn. Wasn't being of unknown quantity part of their threat though? The impression I got was that they were massively under-estimated as a force by all but Leia. I certainly never thought SKB was the full extent of their operation, not even in TFA.