Whatever the story it was a pretty ballsy move. You gotta hand it to those Germans , he probably read the Cessna manual cover to cover and in their typical black or white logic figured there is no reason why this aircraft can't make this flight and just went out and done it, the North Sea be damned. Somewhat similar to the Hess flight to Eaglesham, another exceptional feat of airmanship.
If I'm not mistaken both Rust and Hess went a bit radio rental after their flights perhaps that explains there ability to carry them out in the first place.