Are they really rivals to Nostr tho? 

I guess the answer differs based on what you want to use Nostr for. Personally, I don't see either of them offering foundational building blocks for the new Internet. 

IMHO, only Nostr provides self-sovereign cryptographic identities, web of trust, communication layer, and persistence layer - all of which are truly open and permissionless. That's why I think that Nostr is inevitable.