The updating is now done via their latest app without the need to visit a dealer or buy a cable. I can’t see any reason why normally controlling via a bike computer should prevent this.
My 2 or 3 year old Ribble bought second hand shows as having latest versions...
For anyone getting worried about firmware updates, please note this is now done on the current app without the need to use a dealer or buy any cables.
https://mahle-smartbike.com/firmware-update/