Bluetooth vs Wi-Fi Direct vs mesh networking
Three different local-communication layers. None of them is the internet and none replaces the others.
Direct answer
Bluetooth: a short link. Wi-Fi Direct: a Wi-Fi link with no router, usually more throughput. Mesh: hops across several phones on the same app. offChat can use all three on Android; they are not equivalent.
When each one matters
Two friends at the same viewpoint: Bluetooth is often enough. A slightly more open field and compatible phones: Wi-Fi Direct can help. A long queue with several KingFy phones on: the mesh may relay.