Why not?
Film, especially those films like Gone With the Wind and the like were shot in hi-resolution formats...sometimes 70mm film, but usually 35mm.
The 70mm film has a much higher resolution that even an HDTV at full 1080p resolution can display.
The 35mm film print is close to current HD standards, but still (I believe) has a higher resolution than even a 1080p image. And in the case of AppleTV, 720p is the limit so you are absolutely getting a better image via an HD 'scan' of even old B&W movies.
The data is there, the 'pixel count' (if you want to call it that) of even old B&W movies is of 'HD quality' for sure.
Of course film quality and graininess will come into play...