Show me exactly in the source code where this happens.
Discussion
Oh I forgot nostr is only for CoMpUtEr ScIeNTiStS - normie users need not have opinions and user experiences don’t matter because ShOw Me WhErE iN tHe CoDe!
It says, in the Overview in your link, that the clients get their feeds from the server's interface, not directly from relays.
This is not some sort of secret.
It also caches the media, and replaces them in the rendered events, so that you aren't seeing the original media data that the event contains, but rather their data set.