I was about to say that it's crazy they wouldn't just put up blank red banners and use CGI to add the swastika, but then I realized that the movie being referenced was released in 1965.
The only place where chroma key would help is a stationary flag getting partially obscured by a moving object. Otherwise, it’s more effective to just paste a drawing of all the hakenkreuze over each frame using decades-old subtitling technology. However, even stationary shots with non-moving flags (where the drawing can be reused) could be problematic because of picture jitter.
So what I meant was: the technique they would need was probably not chroma key.
For a stationary establishing shot, they probably could've pulled it off with paint on glass. Then just avoid shooting more than the bottom of the banners.