I don't think the discussion was ever about strict legality, it was about what may happen in the event of an accident involving a bike capable of running at speeds of 30 MPH on an electric motor. The consequences for the rider would differ depending on whether that bike was registered/insured etc. as a moped or not. If it were, and no-one was hurt then I suspect the police would leave it to the insurance companies to sort out. If the bike were not registered/insured the rider may be prosecuted for several offences, receive a serious fine and points on their driving licence, even if the other part were at fault. If there were damage or injury to a person or vehicle then the injured party could also claim in the civil court against the rider, who, not having insurance, would find it difficult to defend themselves.
I suspect that the marking on the rear light would not entertain anyone for long.