An electric motor is classed as an engine. (the hybrids do use both battery and petrol depending on the circumstances at the time so both engine's) The wording in the road traffic act is a mechanically propelled vehicle whether it be battery, combustion, rotary, petrol, diesel, LPG or anything else. I don't know what they are talking about on that program but it doesn't appear in the road traffic act.
That's why these little electric scooters require tax, ins, licence, lights etc to use them on a road. Road also includes the pavement or any other part of the highway.