Wait, they think they had a green screen big enough for that in the 60s? The sheer size of the set they're imagining is wild. They'd need a soundstage bigger than anything that existed back then.
So, I have to laugh at myself because I struggle with a tiny green screen for online meetings. As @the_thea said, the set would need to be bigger than anything back then. How did they think they could hide a soundstage that size? The money and people involved would make it impossible to keep quiet. I mean, do they think everyone in Hollywood was in on it? My cheap webcam greenscreen falls over if I sneeze, so the idea of a moon landing one is just funny.
Actually they wouldn't have used a green screen back then anyway, that tech wasn't ready. They'd have used front projection like they did for movies at the time, think 2001 space scenes. So the whole soundstage size problem is still real, just a different method.