The witnesses

I have always been fascinated by the claim that director Warren Beatty started to film the witnesses in the early 70's. Like he had a grand-plan and a vision all along.

But somehow I doubt this. In the film all the footage of the witnesses look the same (clothes-lighting-film stock). And I have never ever seen anyone mention or give proof that he started filming the interviews before 1979. Is there any?

On Imdb it says: "Warren Beatty began filming interviews with the "witnesses" in the early 1970s."

On Wikipedia it says: "Beatty filmed interviews with a group of men and women, referred to only as "The Witnesses", as early as 1971"

Why does this feel like these claims are based on no evidence whatsoever?