Earlier quoted context omitted.
This is not true. Third party PDSes are fully supported by our app view, and our app view generates timelines for all the users on those PDSes.
What does this have to do with third party app views?
Federated PDS-s (which is probably the closest to what people mean when they say they want to federate on bluesky) would not need to reconstruct timelines if their users use the bsky.app appview.