Sure, but that's just spam. It's true for the whole of Nostr and the old nip04. Any key can send you a DM. 

But a Sybill attack is about using fake identities to gain majority power. The fake identities on Nostr don't give the attacker extra power/influence in the network.  

 Ok sybill was the wrong word. I'm just saying it's worse than normal spam because you can't filter it until you've spent resources to decrypt it, and the server can't filter it for you either. 

 Correct. Which is great if you don't want servers censoring you. :)  

 Touché