To be honest - I passed info to @urszula and I wrote it a few times on the forum - it is possible, but not yet implemented. (also mentioned here Reviews of Mudita Pure)
TL;DR it is complicated to implement and pretty slow
To be able to tether via Bluetooth we have to reimplement the way we’re handling modem connection, as for now we are using is USB port directly during tethering - Pure is a USB passthrough for modem. Moreover, during initial tests of the Bluetooth stack, we’ve got a throughput of about 1Mbps - ain’t much. I’ve passed those requests about Bluetooth tethering further but we have to focus now on more important things such as fixing bugs that you guys have found