I think they saw the White Walker plotline as a necessary evil and just didn't want to introduce more than they had too to get the big battle and I imagine they knew this early on. They were more invested in the political machinations than the magical ones. So, they invented the Night King to just give a face to the White Walkers and an easy thing for the audience to identify as the villain and that was sorta it.
They didn't wan magical dragon horns that may or may not kill people and they didn't want secret burning candles and ancient texts being discovered at the Citadel and some strange Faceless Man plot and all the other stuff that I believe will go into the Wall coming down.
They wanted it to be as simple as possible so they could move on to what they cared about more.
I don't think George held stuff back. I can't imagine they'd have let him.
I just think that the best explanation for why the last seasons seemed so mediocre in comparison to the first 3 or 4 was because they cared about one half of the plot and George cared more about the other.
I've said this before, but I believe they flipped the ending around. That Daenerys loses her dragon before she goes North. That she burns King's Landing before she goes North. That the fight with the White Walkers happens after the battle for King's Landing.
And that a lot of the goofiness that happened at the end was because they changed the order. They wanted Daenerys to be the Final Villain, but did nothing to actually set that up beforehand.
I'm ranting, but just seeing the way the last three seasons went, everything that happened was done to get to Big Battles. Whatever needed to happen to get to the Three Big Battles, would happen, no matter how absurd.
I think George told them about Daenerys setting fire to King's Landing and they got excited and that's the way they decided they wanted to end the show. They just...didn't think about how to set it up in the meantime.
A few reasons I believe this:
1) Maybe I'm wrong. But I can't imagine George is gonna pull the rug and make the White Walkers some secondary, afterthought of a threat. I do think they are the primary evil and will be the final fight.
2) D&D tipped their hand about not giving a fuck of what happened north of the wall when they left bran out of an entire season. And when he was reintroduced to us we got absolutely zero of the Bran we see in ADWD.
All of the visions Bran has in ADWD, all of these historical things he sees, many that we readers still don't know to this day, were out. Instead Bran was used to reveal Jon's parentage and....that was it. That was all. And then be King.
Bran is the very first chapter in AGOT. He is the most powerful character in the entire series. His chapters in ADWD, particularly his last, are DARK. He's seeing dead dreamers before him, he's eating some hallucinogenic type of paste (that may or may not contain Jojen's remains), he's having weir wood visions where he sees an ancient people making blood sacrifices to the trees and he, while he's dreaming, is drinking the blood of those sacrifices.
We got absolutely none of that.
We also got absolutely none of the shit happening in Oldtown. Burning candles and Faceless Men assassinating assistants and taking their faces. Instead, we got Sam reading one line about an annulment and that was it. That was literally it.
So, this was long-winded. But in my humble opinion what is happening in Oldtown in the books and what is happening North of the wall are the two most important things as it relates to the White Walkers, the Wall, and how it comes down and in the show we had zero of that.
So, to answer your question, I think George told them his plan, or at least some basic outline of it, and they just took bits and pieces to make their own vision work. They wanted politics to be the end point, and introducing horns and more and more magic would have distracted from what they really wanted to do.
Maybe George proves me wrong, but the fact that even in this book he apparently refers to no show scenes after "Hold the Door" I think, is telling.
Re-reading, I was actually wrong. He simply states he told D&D who would be King. He didn't actually say he told them Bran would be. So, maybe I'm reading between the lines, but I do think his ending and theirs will be fairly different.