alt Hacker NewsImage generation is nothing like AI code generation in this regard. Copying artist style is one of the things that is explicitly quite easy to do for open-weight models. Go to civitai and there are a million LORAs trained specifically on recreating artist style. Earlier on in the Stable Diffusion days it even got fairly meanspirited - someone would make a lora for an artist (or there would be enough in the training data for the base model to not need it) and an artist would complain about people using it to copy their style, and then there would be an influx of people making more and better LORAs for that artist. Sam Yang put out what was initially a relatively tame tweet complaining about it, and people instantly started trying to train them just to replicate his style even more closely.
Note, the original artist whose style Stable Diffusion was supposedly copying (Greg someone, a "concept art matte painting" artist) was in fact never in the training data.
Style is in the eye of the beholder and it seems that the text encoder just interpreted his name closely enough for it to seemingly work.