GRRM has taken to NotABlog (His official medium to release statements and correspond with the fans) to answer this question after a fashion:
And I’m writing. Winter is coming, I told you, long ago… and so it
is. THE WINDS OF WINTER is very late, I know, I know, but it will be
done. I won’t say when, I’ve tried that before, only to burn you all
and jinx myself… but I will finish it, and then will come A DREAM OF
How will it all end? I hear people asking. The same ending as the show? Different?
Well… yes. And no. And yes. And no. And yes. And no. And yes.
I am working in a very different medium than David and Dan, never
forget. They had eight hours for this final season. I expect
these last two books of mine will fill 3000 manuscript pages between
them before I’m done… and if more pages and chapters and scenes are
needed, I’ll add them. And of course the butterfly effect will be at
work as well; those of you who follow this Not A Blog will know that
I’ve been talking about that since season one. There are characters
who never made it onto the screen at all, and others who died in the
show but still live in the books… so if nothing else, the readers will
learn what happened to Jeyne Poole, Lady Stoneheart, Penny and her
pig, Skahaz Shavepate, Arianne Martell, Darkstar, Victarion Greyjoy,
Ser Garlan the Gallant, Aegon VI, and a myriad of other characters
both great and small that viewers of the show never had the chance to
meet. And yes, there will be unicorns… of a sort…
Book or show, which will be the “real” ending? It’s a silly question. How many children did Scarlett O’Hara have?
How about this? I’ll write it. You read it. Then everyone can make up their own mind, and argue about it on the internet.
It seems from George's statement that there will be some parts of the books' ending that will be similar to the show's ending but in other parts it will take different roads. Of course as George warns, he works in a different medium than TV so he'll be throwing in additional chapters and plots if needed. And if he likes those plots? Well it won't be first time he completely changed the story, he's known to change everything if it doesn't meet his standards and dumps hundreds of pages in trashcan if they take his characters on a journey he doesn't like. For example as cited in the article, he wanted to end the books after Red Wedding with Arya and Jon hooking up at the Wall and Tyrion acting like Jorah Mormont. How far have we come from that ending?
Not to mention, his warning about the Butterfly effect says quite a lot. In Chaos theory, butterfly effect is essentially that small changes in initial state of a system can result in much larger changes at a later state. This is something that is consistent with GRRM's writing style. If you compare GRRM's style to let's say JKR's style, you'll see that JKR is an architect, she knows what she wants and she builds the story towards that end. GRRM is a gardener, he plants some seeds, lets them grow naturally. If he doesn't like the growth, he prunes some. If he still doesn't like it, he cuts the whole thing down and starts again. He might have had an idea what the garden would look in its final shape but that doesn't mean he will stick to that shape in case something better hits him.
In other words, it is possible that this was more or less 90% GRRM's ending 8 years ago. Is it necessarily true today? Well… yes. And no. And yes. And no. And yes. And no. And yes. And since GRRM says his ending can be different, I'd say he's not contractually obligated to abide by that ending.