Not sure how it's implemented, but the main point of the product is to take a USB Midi signal and convert it to Bluetooth MIDI. Product page is at the link below if that provides any more detail. As I described, it works somewhat with FracPad and the AX8 initially but then things drop out.
This device is not a MIDI over USB device. Use the Yamaha UD-BT01 with the AX8