The FlashTune cable is not just a cable. There is some electronics in it.. I believe they are FTDI cables. At least installing the FlashTune software installs FTDI drivers.
https://www.ftdichip.com/Products/Cables.htm
These cables have an internal USB uart to send/receive USB protocol data to/from your computer, and a can be programmed to control the output pins in various ways. The cables can be programmed with a non-changeable serial number to let software identify the cable.
The FlashTune software expects to see a FlashTune cable. And information about the ECU is stored either in the cable, or in some FlashTune data on your PC, to lock it to paid for targets. If you knew enough about what FlashTune does with the cable, I suppose you could make your own. You would also need the motorcycle specific connector(s) for the ECU end.
Old FlashTune versions didn't lock the cable to the ECU, but they (FlashTune) won't give you an old version. All FlashTune versions I've used will automatically update themselves if they have an internet connection. A few years ago, they changed the software so it won't run without an internet connection.