It is not a decoration. Karpathy juxtaposes ChatGPT (which feels like a "better google" to most people) to Claude Code, which, apparently, feels different to him. It's a comparison between the two.
You might find this statement non-informative, but without two parts there's no comparison. That's really the semantics of the statement which Karpathy is trying to express.
ChatGPT-ish "it's not just" is annoying because the first part is usually a strawman, something reader considers trite. But it's not the case here.
Indeed, I was probably grumpy at the time I wrote the comment. I do find some truth in it still.
You're right ! The strawman theory is based.
But I think there's more to it, I find dislikable the structure of these sentences (which I find a bit sensationnalist for nothing, I don't know, maybe I am still grumpy).