The headline before/after image is astonishing, almost in-credible. I can't see how the left image was restored into the right. It looks like there is substantial new detail on the right that I can't see anywhere on the left.
I can only assume that the image on the left is a low resolution scan produced for this web article, and that there must be a much better scan somewhere else.
It must be. The amount of detail is incredible, and even trying to extract data from the before picture, it doesn't come close to what you see in the newly processed image.
Absolute least they did is to rescan original film with newer type of scanning process/device into higher resolution and bit depth „digital negative”. You cannot replicate that from low quality jpeg image.
It sounds like they took the original film negatives and rescanned them. You can scan them at varying levels of light intensity to get more detail in the highlights/lowlights. By comparison a single compressed digital file is going to have a limited dynamic range.
I can only assume that the image on the left is a low resolution scan produced for this web article, and that there must be a much better scan somewhere else.