Yes that's what it does. Also saves a ton of bandwidth for mobile devices since they only have to read from one server instead of a dozen or more. It's currently indexing the top 100 relays, but I'm updating that tonight (doubling it) now that more relays are coming online. Writes go out to all configured write relays in the usual way, but of course writes are a tiny fraction of bandwidth and CPU usage to create the signatures on those.