I think these guideline are built into the llms. After all with llms, any decision you don't take will be taken for you. For a so called slop site (Simon's words, not mine :P) I don't think they specified a style so the llm just decided the default it knows. One day I will run experiments (when I can afford it) trying to identify the "taste" of different llms.
I told it "Design aesthetic should be technical docs, minimal flare" - full prompt here: https://news.ycombinator.com/item?id=49504625#49505116