The author missed the big red "Turn Siri Off" button at the bottom of the same preferences window that they're using as proof that it can't be turned off. https://imgur.com/a/hB5fWLS
Note that the alert isn’t about being able or not to disable Siri. It’s about Apple Intelligence. Those are two different things. You can disable Siri AI and you’ll still have Apple Intelligence in other OS surfaces.