I'm guessing the old 'jack test' boasting began way back in the earliest days of Speedster replicas when back yard privateers were trying to build their own by slapping a fiberglass body on a shortened pan, with maybe some uprights to support the front and back, and calling it 'done'. Some of them must have tried to get away with not adding a steel perimeter frame.
It looks like the shops that became successful businesses and went on to make cars in volume quickly realized that wouldn't work.