AT model is very different from Mastodon or email. It’s much closer spiritually to RSS and plain old web.
Mastodon is “many copies of the same app emailing each other”. There’s no global shared view of the network so you can’t have features like globally accurate like counts, shared identity, global search, algorithmic feeds across instances, etc.
On the other hand, in AT, the idea is just that apps aggregate information from different repos. So each application’s server has information aggregated from the entire network. Everybody sees the same consistent information; apps exist to separate experiences rather than communities.
For example, Tangled (https://tangled.org) and Leaflet (https://leaflet.pub) are AT apps, but they’re nothing similar to “mastodon servers”. These are complete apps that implement different experiences but on the same global network.
Crucially, normal people don’t need to “buy into” the protocol stuff with AT. Most Bluesky users don’t know what AT is and don’t care about it; they’re just using the app. There’s interesting crossovers you can do (each AT app sees each other AT app’s public data) which do bleed into the user experience (eg my Tangled avatar is actually populated from Bluesky) but overall apps compete on their merit with centralized apps.
Hope that makes sense. See https://overreacted.io/open-social/ for a longer article I wrote about AT with visual explanations.