He was just a lying shit. And thankfully we can also ignore Genie. He's not feral though, he's sounded by speech (mostly from the tv, but speech is speech). I think the best comparison might be how deaf kids learn speech, except without the "accent" since he can (probably) hear himself. Even that's not right though, because deaf kids witness normal social interaction and how conversations work. Nobody in that house talks to Atlas, or really anyone else.
Luna is grim, but I feel like a 3 year old boy stuck inside in pajamas, ignored, and stuck in front of the tv while dad works out where it's loud and mom spends all her time on bs for her spud, or on the baby. It's a good thing he really wants to help Luna's "therapists", I'm not worried about sibling rivalry with the baby. It's just fucking sad that this kid's only interaction outside his fucked up family is because the eye-fucker brings people in to "help" her unhelpable child.