I came here to comment "We all saw this coming, right?", but apparently some people are actually surprised by the reveal.
I saw it coming, yeah, but it's still interesting to see. I'm wondering where he'll go from here. He's just realized that everything he thought he knew was a lie.
But dat wall though. It seems to be affectionate towards its host, but isn't able to restrain its hunger. I'm wondering if the wall is a metaphor for Kim himself as a reader. Eternally devouring content to satisfy itself. Perhaps the wall is a part of Kim. A part of him that he created to protect himself? It always seems to show up when he needs help, using the pages of the stories he read and used as escapism.
The real question will be whether he can get his mom out of the wall or not. Normally I'd say no, but on the other hand, I can't remember a single time he actually lost, and part of why I like this story so much is seeing how he wiggles out of all these bullshit situations to win despite how the deck is stacked against him. I don't want to see him lose; the whole theme of the story is rejecting narrative conventions after all. So I'm hoping he ends up saving her somehow.
Though, a thought occurs to me. What if the guy who wrote the original novel is Kim himself and he wrote it as escapism and forgot just like how he forgot he murdered his dad? I wonder if that scene where the manga artist had to let her own work go was foreshadowing something like this.