Realistically the legs are probably just actual drawings. Another likely reality is photoshop, to apply a noise filter, which would do a lot of heavy lifting in blending the details together to make it feel realistic.
Took me a minute to browse through their portfolio and twitter account to find that they have posted sketches, gifs, and even memes about their drawing process. Semi-realistic anatomy and hyper detail appears to be a large focus of their work. People need to stop accusing everything of being AI, it makes only you look the fool and is disrespectful to people that work hard to make their pictures look good.