This isn't accurate. All of the relays do some filtering, so the algorithm is just a level lower. You don't know what was sorted out before it got to you, even if it was a bot or spam from one of the npubs you follow. The relays would eventually collapse under notes, if they didn't curate, at least a bit.

That is a simple attack vector: just flood them with nonsense notes saying "Happy Birthday!" or containing kiddie-porn, or whatever, and shut down the server.