I haven't seen this movie in a long time, but I always thought of JC's version as a sequel to the 50's version. The main thing that leads me to this is the scene where they are viewing the videos/films the Norwegians made of the ufo. those clips look exactly like clips from the original movie.
The 50s version takes place in the 50s, yes? The 80s one takes place in the 80s. Unless the 50s cast were all secretly Norwegians and hung out in Antarctica for thirty years before all dying horrible deaths a couple days before the start of JC's film, I'd say sequel doesn't really fit.