I have a friend who is a professor of sociology. She has written published papers on how jokes work. I had told her jokes and thought her a tough audience because she would sit there with a polite smile on her face, the very type of some one who doesn't get it, that makes you wonder if you're telling the joke right. Then she had me come in to tell her class jokes to illustrate what's funny. I think she is fascinated by what makes people laugh because she doesn't really seem to find anything really funny. Long study has made her able to recognize the elements that make something funny, but it never touches her except on an intellectual level.
That's how I feel about this documentary about Mel Brooks and Carl Reiner doing their 2000 Year Old Man Routine. There are comedians talking about how much they love it and a rabbi talking about the social conditions from which Jewish humor arose. Some talk about the relationship between Brooks and Reiner and the importance of the straight man in humor.
None of these interviews will give you anything more than a technical appreciation of this film's subject, which is presented in bits and pieces. While it may make it clearer to you why others find it funny, it won't make it funny. That has to come from you.
So, my advice is to not look at this movie. Go listen to one of the versions of the routine. If you enjoy it, if you laugh, if you wonder why you find it funny, then you can go watch this documentary.