Thanks for the video. I ended up watching the whole thing. My interpretation is a little different than yours: I think George is saying that there's no point in having a key longer that 56 bits given that the goal is 56 bits of security, but he's vague about where the requirement for 56 bits of security came from. In any case, the video certainly supports my larger point that the idea that NSA would sabotage a crypto standard was mainstream within crypto circles, even in the '70s.