Rinzer doesn't really "say" anything on the subject. His books are pretty much definitive accounts of the inception, development and creation of the Star Wars movies via a vast collection of interviews--both old and new--from the people who were directly involved at every step along with period correspondence and relevant ephemera. It's less of a "making of" and more of a biography, though of a film instead of a person.
I cite this because you seem struggling under the misguided assumption that any creative endeavour where the creator doesn't seal themselves inside an airtight chamber and eschews all support, advice and feedback is somehow less than valid. As if the person driving the whole thing is somehow coasting of the efforts of others. The reasoning that the only explanation for the perceived difference in quality from two sets of films made decades apart is the unsubstantiated lack of "outside help" is spurious at best.
Of course Lucas had help. That's never been in contention. He was working on various versions of that initial script since he started making 'American Graffiti'. The whole time he would bounce ideas and seek the advice from his circle of friends from film school as they sought his feedback from the various projects they were working on. And yes, he had help when making the Prequels too. Indeed a whole company and several subsidiaries who's entire purpose was to help him make movies.
