I think it's a shame it's been missed out, but not so much because it is an American flag, more because I'm an Apollo enthusiast and I would be quite happy to watch a recreation of the entire EVA.
The film, however, is not called "A history of Apollo 11", so I get where the film makers are coming from in leaving it out. Had this been "Last Man" and they left out Geno's flag ceremony, which he declared to be the proudest day of his life, it would be different.
If the flag wavers are disappointed about the lack of one to wave, they need to fund their own movie.