Can you do that on the current Facebook? You can as long as Facebook gives you API access or something like that. It's the same on ATProto.

I could be wrong, but I think he's getting at most users won’t bother switching clients just to see one person’s posts and they’ll most likely stick with the mainstream app (in the current case of at proto, Bluesky) - and in this case they won't be able to see your posts from another app because fetching data from your pds is prohibited by the main app overall, no matter where it comes from. Your friends would need to use the client that pulls from your pds to see them not a knock on at proto but the concept of bluesky being a dominant app on at proto