The only way it could have been a medieval forgery was for them to have obtained an authentic cloth from around 33ad and have full knowledge of the exact way a scourging and crucifixion was carried out and then killed a man in an extremely specific way. And then have some way to produce the actual image using the body of a man to create the bloodstains... then they'd have to remove the cloth and do whatever process to produce the image and get the cloth put back on perfectly in alignment.

If such technology existed to produce the image so perfectly... there would have been more practice attempts circulating and potentially others discovering the method and then using it for commercial purposes. We don't even have a viable method that could produce the image. They'd have to have predicted technology far surpassing would could have been understood by humans at the time.

So either the shroud is a paranormal phenomenon, or was made by someone who could see the future.