How can a movie from 1987 be better in HD than SD? Does that apply to all movies that were recorded on old-style film?
Because the resolution of film is better than the resolution of standard-definition TV. It is difficult to define the resolution of 'film' since it is not a digital, pixel based media. But film is designed to be sharp even on much larger screens than any television. We can however use the resolution of digital cinema projectors as a good proxy figure.
So to compare the resolutions:
As you can see from the figures above, the resolution of film exceeds the resolution of SD television by some margin. The resolution of HD TV approaches that of digital cinema.
So a transfer of a movie shot on film to HD TV is going to result in a more faithful copy than than a transfer to SD TV. So a movie shot in the 1980s (or even the 1930s) is going to look better in an HD transfer than an SD one.