I don't think it has a 'mechanical' diode
The gearbox is a planetary but I see no clutch or reverse ratchet (aside from at the pedals/chain). Looking at the Control Board schematic:
No Reverse- or reverse movement, based on diode 1N5401 D1. They say the diode smokes if it's pushed backwards at anything faster than a crawl.
It's weird with Sinclair stuff, it always has a lot of science when you dig in, if you can push aside the practical aspects being a bit weak.
I think it's the first 1980's Coulomb counter I've seen, in the Fuel gauge. Lots of measurements on the speed, torque, range, acceleration, hill climbing etc.
Like everyone went along with the Emperor's plan- not saying anything about his clothes. Perhaps for the business and money you'd do this kind of work.