If you compare the AI image that was used with the image that one the price after the artist enhanced it to that level you could argue that paintings from sketches are not copyright-able
Well if the sketch was made by the artist then no you can't, and if the sketch wasn't then the copyright board has a right to know, and he didn't disclose the original image.
He's allowed to copyright it as a collage, just not claim ownership to the source images.
When you say a painting from a sketch, what do you mean? Is it a sketch from another artist? If so, you can still copyright the painting, you just can't claim ownership of the sketch, because you didn't make it.