Image on Github is JPEG made from RAW. Since RAW file has more dynamic range and contains a lot more information than JPEG you can take that photo in an editor and crank up the brightness. You will get a noisy image but it will be a lot brighter and will probably resemble the image with the high ISO in the middle. Then in an editor you can apply some de-noiser to get results similar to the last one.
So presumably this neural net more or less does it for you.
The *PNG is there just to show the results produced by the CNN, if you watch the linked video they do exactly what you are suggesting and then compare both results.
