How is this better than signal?

Briar is probably the goalpost they have to overcome. Internet plus mesh Bluetooth

And Bitchat is really closer to Meshtastic. It can use LoRa repeaters (based on a cursory look, I could be wrong) while Briar seems it's either direct BT messaging or internet, no mesh relays. If Briar just allowed encrypted BLE repeaters, it would be the obvious winner.

I might have this wrong with Briar, but it seems like nothing does this exactly as would make sense: Decentralized as prioroty routing, internet routing as backup.

How it works - Briar

Briar is a messaging app designed for activists, journalists, and anyone else who needs a safe, easy and robust way to communicate. Unlike traditional messaging apps, Briar doesn’t rely on a central server – messages are synchronized directly between the users’ devices. If the Internet’s down, Briar can sync via Bluetooth, Wi-Fi or memory cards, keeping the information flowing in a crisis. If the Internet’s up, Briar can sync via the Tor network, protecting users and their relationships from surveillance.

(briarproject.org)

The briar mesh is via peer to peer Bluetooth / wifi connections but it is a mesh via briar message passing

In more technical terms, each forum/group/blog has a distribution graph where the nodes are subscribers and the edges are a subset of contact relationships (specifically, the subset that have chosen to reveal the subscription to each other).

So if your friend group is at a concert and the internet is down you can have a forum where y'all call still communicate, with the caveat that only people in the group will mesh/proxy messages once in range.. the benefit of this is that nobody outside the group can see the message activity, if one person makes it to the internet all messages will sync and when they go back to the concert the offline members will see internet messages.