It's not "other stuff", it's stuff other than Nostr.
It is as much Nostr as Google Drive is.
Nostr is a protocol. Nostr.build is an image host like many, it just happens to have "Nostr" in the name.
> You do realise the relays (the R in nostr) are just servers, right?
They are Nostr servers.
Not all servers are Nostr servers. Only those that operate through the Nostr protocol are.