No, they are not. There is no federation between relays dictated by the protocol, if that's what you mean. Relays are just servers maintaining a database. Just like SN. The only major difference might be that they use websockets instead of HTTP for communication.1
What makes nostr decentralized is that there are many relays and clients can choose to which relays they want to connect.
However, I trust that the SN ranking engine/algorithm (whatever you want to call it) could be run on over a SN relay somehow, notes got zaps too
Footnotes