I think this compares AT proto PDSes to Fediverse instances. In many ways this actually underplays just how grossly centralized AT proto currently is, since some of the components are 100% centralized still. (Whereas Fediverse instances, all downsides considered, are at least self-contained fully-independent instances.)
Which components are you referring to?
In case of Bluesky, there will also only ever be a single instance of the App View. As far as I am aware though in practice there's really only the official relay or indexing services either.
Why can't someone run another App View?
This one isn't an AppView (it's a frontend for Bluesky's). However,
https://blacksky.community/ and
https://zeppelin.social/ are full AppViews.