I agree in general but the web was already polluted by Google's unwritten SEO rules. Single-sentence paragraphs, multiple keyword repetitions and focus on "indexability" instead of readability, made the web a less than ideal source for such analysis long before LLMs. It also made the web a less than ideal source for training. And yet LLMs were still fed articles written for Googlebot, not humans. ML/LLM is the second…
Blog spam was generally written by humans. While it sucked for other reasons, it seemed fine for measuring basic word frequencies in human-written text. The frequencies are probably biased in some ways, but this is true for most text. A textbook on carburetor maintenance is going to have the word "carburetor" at way above the baseline. As long as you have a healthy mix of varied books, news articles, and blogs, you're fine.
In contrast, LLM content is just a serpent eating its own tail - you're trying to build a statistical model of word distribution off the output of a (more sophisticated) model of word distribution.