I suspect they are replying to this part of the article: "What you actually want to say is: “an element is effectively-dark if it has data-theme=”dark”, or if it has an effectively-dark ancestor with no effectively-light ancestor in between.” That’s a recursive relational definition. CSS cannot express it. CSSLog can:"

The entire article doesn't seem to mention the existence of :has() which is rather surprising given how recently it was written. Not even in the footnotes.