It's very clearly written with ChatGPT's authorial voice. The very prominent sprinkling of bold intensifiers, the frequent organization of things into lists to make arguments, the use of emojis in the full context pages, the overwhelming use of em dash to structure sentences, overuse of "it's <whatever> — not <whatever>" type cliches, etc.

It's likely because he's not a native English speaker, hence the perfect grammar and spelling in the contents of the gist, but the obvious grammatical error in the title ("Why I leave Ruby Central") that he likely typed himself.

Given his reason for doing it (non-native speaker), I don't think it's fair to really criticize him for it, but if you can't spot this as AI writing, then you might want to do some research on how to do so.