The Redditor used a technique developed by Google called DreamBooth, reimplemented for Stable Diffusion, using only 32 illustrations from the artist. Training took 2.5 hours of cloud GPU time at a cost of under $2. He then released the finetuned model for everyone to use.
DreamBooth allows you to introduce new subjects or styles to Stable Diffusion, with leaps in speed and usability happening weekly. I was able to train an AI on my own face in about 20 minutes on Google Colab. It cost me about $0.20. (You can also do it for free, but it's slower.)