He was still wrong, it’s not “Mr. Prime Minister”, the correct form of address for the UK PM it is simply “Prime Minister”.
Addressing the PM as “Prime Minister” outside of Parliament is a recent American style cultural import. Thatcher was referred to as Mrs and then Lady Thatcher (after her husband Dennis became a baronet) whilst she was Prime Minister. Frankly I think that was better. We shouldn’t be giving too many heirs and graces to politicians but our senior politicians have been happy to adopt the kinds of styles that are granted to their American counterparts because it makes them feel much grander.