Depends on whom you ask. It's like how many people believe Spielberg ghost directed Poltergiest. Hopefully the forthcoming Rinzler book on Jedi will set the record straight. Marquand made some interesting movies before and after Jedi, so I don't doubt his directing skills.
skyjedi, Prowse has claimed for years he was unfairly blamed for a script leak during production on Jedi and things got very unpleasant on the set after that.