I'd distinguish verbal inner monologue from consciousness. Many people think on the daily without narrating things to themselves in sentences, without feeling brutish or animalistic. I can plan complex human actions including theory of mind considerations without narrating it as a monologue. It's still very conscious.

Also that reddit post seems like a fake story made up by a teenager as a sort of scifi. It's just too on the nose. Yeah he lived life without understanding that his friends still exist while out of sight... I guess the OP just learned about some developmental stages of toddlers or object permanence etc and wrote a story like this as fanfic.