Yeah, I thought so — and the paper doesn’t substantiate the claim that the IS cannot be used.
Alan Berman
1

One issue in GAN is the tendency of claiming the “state-of-the-art” technology by evaluating a single dataset. So I will not be too focused on one dataset in evaluating the results.

Back to your question, FID will probably work too. There are researches using ImageNet for line art and demonstrate they have a lot of commonalities.