Both the Targa bar and the large rear glass add a bit of reinforcement, Not a lot but some. I think the factory Cabriolet did have additional reinforcement on the inner rockers and some inside on the lower kick panels.
Maybe he just thought that as a Cab it's use would be more as a boulevard cruiser.
Don't get me wrong, I really like the car and am really glad you are restoring it to it's original Ruf spec. What color are you going with?