I guess it is the Hall-effect sensor switch - it either cannot trip itself enough times per second to get to the frequency of switches needed to exceed 60kmh, or it can, and the cable has a problem sending the pulsed signal, as you guessed.
But why. Is it not picking up the ferrous bolts correctly? Is it corroded inside? Is the cable coming out of the speed senor corroded?
The cable has a double rubber protection but tiny amounts of water can get inside and will collect at the base of the cable - its lowest point - where the cable comes down from the frame then bends back up again to the wheel speed sensor mount near the brake caliper. This copper cable can corrode making a poor connection. This bad connection creates an intermittent issue and/or increased resistance, either of which would prevent the cable transmitting the pulsed signal accurately/correctly.
So, if you have the tools, you could remove the rubber protections around the cable near its lowest bend and see if there is a problem with corroded copper.