I do not know for sure, but I suspect the author is using GANs (Generative Adversarial Networks) to do this. There are many GAN versions that create drawings and sketches from images. Here are some references:

• RPD-GAN: Learning to Draw Realistic Paintings with Generative Adversarial Network
• I used a similar approach to generate a sketch of my photo too. large-scale image datasets with learning analytics

I used a similar approach to generate sketch of my photo too. Here it is.