There's some overlap with P2P Matrix (which isn't yet done but is in the works).
Briar was built for P2P from the ground up, so presumably they planned this better.
This isn't a criticism of matrix. Matrix is great and I use it everyday but gotta give credit where its due.
I see no reason to think P2P isn't a goal of Matrix either; the reason it's not P2P now is that federation was easier, but the long-term game certainly looks to be P2P with work on cryptographic, portable identities and Bluetooth mesh networking in the form of Pinecone. I've also seen arathorn confirm this on HN.
I hope this will change with p2p matrix...