Explaining ATProto to people used to ActivityPub is as hard as explaining ActivityPub to people coming from centralized social media because it's just as big a conceptual shift.
Twitter/Facebook/etc->AP split one center into multiple centers. App and data are still bound together.
AP->AT splits app and data. You choose your data host, and then you choose your app. Most apps will bring data hosting along and you won't have to think about it, but the option is there: the credible exit.