Adding a line where you want there to be a shadow in the output seems like something you could learn from trial and error when messing with a model. It somewhat weakens the accomplishment of the paper if the sketches aren’t drawn by naive users, but it’s a lot more defensible than generating the input like you suggest.
Agreed. It just looks a bit strange and doesn't help to instil confidence in the paper. My first guess would be that they've used a reverser for the learning process somehow. As it's a preprint, hopefully comments like this will help them to strengthen the paper and release their code!
