No it was Han *in* TFA that said he walked away. Does that mean Han had to be proven right? The Avengers thought that Doctor Strange unheroically gave away the farm and even stated as much until it was revealed that he in reality he had a long term plan to defeat Thanos that they weren't immediately privy to. TFA did indeed imply that Luke simply gave up but that doesn't mean TLJ had to confirm it.
It wasn't just Han saying it. The whole premise of TFA proved it. From Lor San Tekka, to Leia, to Han . . . and more importantly, to the fact that it was very clear that Snoke and Kylo were building an army that would go on to destroy five planets, the Senate, and the whole Republic with it. And all with no Luke anywhere to be found. No Luke rushing in to save the day like he always had.
Luke was the last Jedi (aside from those he was training). The whole purpose of the Jedi was to keep the galaxy safe from evil/Sith. No matter how you slice it, Luke didn't intervene (as a Jedi would) to confront Snoke or Kylo. He didn't get involved at all. Han didn't need to say that he "walked away from everything" in order for us to understand that he did do exactly that. All established by TFA, not TLJ.
TFA was all about how the First Order had become enough of a violent presence to necessitate an entire Resistance movement that was heavily outnumbered. So Leia felt that she needed a Jedi; she needed Luke Skywalker, and she spent resources looking for him. Because he was missing; as in willfully ignoring the conflict and the threat. Then showing up on the edge of a cliff, seemingly able-bodied, and . . . doing nothing.
Well again, a good writer will explore all his options. If he wasn't cut off from the Force we know that Luke would have sensed Starkiller blowing up planets and most likely Han being killed as well (unless that's only a family thing.) So would he have had to jump in an X-Wing that *instant* to help out? No, just like Old Ben didn't go charging out of the Falcon when it was captured by the Death Star. He devised a plan, strategized, and so forth. Luke could have been doing that when Rey arrived (remember it was only a few hours after the Hosnian System had been destroyed.)
And what would Luke have been doing for the intervening years after the destruction of his temple? Wouldn't it make sense to be strategizing with Leia (his sister, who was the only other remaining Force-sensitive, and who led a military presence of her own)?
Luke is on the cliff, processing the fact that Han just died, turns around with tears in his eyes to see Rey, she offers the lightsaber and then he gets to have his "let's go get this son of a *****" moment. Maybe there are other pupils and maybe there aren't. Since even TLJ implied that his X-Wing was inoperable they had a built in reason why he couldn't help anyone right there. No need to bring in the whole "closing himself off from his friends" element.
How would you go about explaining why Luke didn't have the "let's go get this son of a *****" moment earlier? Because of an inoperable X-Wing? How stupid is Luke not to have an astromech droid with him to help him do repairs? And why wouldn't he have reached out to Leia? Why not use the Force to send a message to her, or have one of his Force ghost buddies do it?
Or maybe even just say that like the funky cave in Dagobah the entire island itself does wonky things with the Force. Like the actual island is a "Force black hole" that can't be detected (hence it being a perfect hiding place for the ancient texts) but that also cuts off whoever is there from the rest of the galaxy as well. Then they could have said that in order to get the texts and possibly learn sweet new powers with which to use against the FO he had no choice but to be temporarily cut off from the Force, something that he didn't intend.
For me, having Luke exile himself from his sister and her Resistance (for at least a couple of years) in order to "learn sweet new powers" would be absurd.
You act as if the destruction of the Hosnian system, or the murder of Han, is what should've compelled Luke to get off of his ***. Leia didn't think so. She had been forming a Resistance *for years* to confront the FO. Why would Luke have been absent (abandoning her) during that time of urgency? To learn "sweet new powers" from some books that he could've just taken with him? I don't think that would go over any better. Not with me, at least.
I just think that if you're writing this movie and you take days, weeks, hell 10 *minutes* mulling this stuff over that there are plenty of scenarios that would explain why Luke was off trying to accomplish something good in any way that he could, even if he *did* feel disenchanted with the Jedi Order.
And I think that's what RJ did. He had to explain more narrative problems than I think you're attributing to the setup from TFA.
As a fan, I spent the two years after TFA thinking through plausible scenarios that would salvage Luke from having simply been a cowardly punk. I always ran into dilemmas when it came to reconciling plot logic. For example, I thought that maybe Luke at the end of TFA could've been a doppleganger of some kind. Rey would discover something like the real Luke having been found by Snoke and kept away in secret (even from Kylo). Episode 8 could've been Luke finally escaping and kicking ***. But that idea sucks too, and for many reasons.
The more you peel the onion of alternative explanations for Luke's absence, the more that onion will stink. I think RJ probably came up with tons of ideas, and chose the least-stinky onion. I'm surprised that you don't see it that way now, but that's fine. Disagreeing with you about this stuff is fun too.
Because they are examples of how his entire life has been rife with bad things happening to him and his loved ones. Also his friends were quite literally "made to suffer" because of *him,* Yoda said as much. So did he go, "oh well I need to take myself out of the equation since people suffer on my account?" No. Did he surrender to Vader without a fight on Cloud City since him being alive put his friends in danger? No. That just wasn't who he was.
In his mind, he wasn't taking himself out of the equation just for failing. He was ending a cycle that seemed to repeat itself too specifically, and he created a delusion for himself that the only way to be sure it could end would be to end the Jedi religion and leave the fight to a new generation who would fight the eternal struggle their own way (hopefully finding a *better* way). But again, he was brought around to a deeper and more sober understanding by the end of TLJ (after explaining his problematic absence established by TFA).