The intention behind "or must be", is clearly to cover potential situations where the driver caught driving whist using a device while not currently holding it, but must have been holding it and using it while driving at some point - much like not being in a position to be in proper control in cases where the driver could not have been in proper control but where swerving, etc. were not witnessed due to the road being straight at that point.
How many devices cannot be operated by voice control, or by touch screen without holding whilst in a cradle?
Taking the argument to its logical conclusion, if a driver initiates a call before he enters the car in a private car park, automatically connects to the car's bluetooth and then locks the phone in the boot before driving onto a public road whilst continuing the call, is this behaviour that Parliament intended to criminalise (ignoring the dangers of distraction from non-hand held calls, which is not specifically addressed)?