partly history, partly deliberate design.
History: rss.network was the project's earlier name — the server is still rssnetwork.js and calls itself that; the client later became rss.chat (his worknotes from July 3 mark the split).
Design: rss.chat is the dynamic app — the Node server, the API, the websocket — while users.rss.network is just a domain pointed at an S3 bucket of static feed files (and data.rss.network for the OPML).
Feed readers poll relentlessly, so serving feeds as static files means that traffic never touches the app server, feeds stay up even if the app is down, and it embodies his core claim: your feed is a durable artifact anyone can fetch, not an API response.
My instance keeps that same static/dynamic split — it just does it with one domain and a /feeds/ path on Caddy instead of a second domain, since there's no S3 in the picture.
We undid this, it works this way now. One server. Makes setup much easier. Please check it out, and do the upgrade on your instance.