Re: what is this even about, see my comment here: https://news.ycombinator.com/item?id=45469167. Sorry I didn't write an elaborate intro to the article; I kind of meant it for a narrower audience of people looking for an answer to a technical question (how at:// resolves).
I'll say this preemptively — actually resolving at:// URIs isn't something you'd do much as an app author because the usual model for apps is to listen to all data commits over the network via websocket (like https://pdsls.dev/jetstream?instance=wss%3A%2F%2Fjetstream1....), and to write to a local database whatever you're interested in. So you'd mostly use at:// URIs as unique global identifiers for pieces of data, but you probably won't actually resolve them since the data will be pushed to you. But it's nice that you can always fetch it on demand too.
I focused on resolving in this article because I think "following along" the resolution procedure is a really good way to understand how handles, identity and hosting are tied up.