That's neither here nor there. We were specifically talking about why/how film can be remastered to "HD" but something captured in digital "SD" can't be "upscaled" to true HD resolution
Yes, just because an image is captured on film doesn't mean it's going to be high resolution. The focus, lighting, composition, subject matter, whether there was any in camera effects like a diffusion screen will affect how successfully a 35mm print can be transferred. This is particularly true of older classics that loved throwing on a diffusion screen every time the leading lady was on screen. Just look at The Sound of Music, "HD" is nearly meaningless when the image is purposefully diffused in order to hide wrinkles and other 'imperfections'. However, that doesn't diminish the general rule that given a scene with the same focus, lighting, composition, etc, the image shot on film will contain more information, color depth, etc, than a digital equivalent.