I don't think Google will ever be able to distinguish texts that are written for people from texts that are written for Google. It's effectively the Turing test, and Google won't pass it.
It's worse than a Turing test: it's trying to distinguish between a honest human (writing content they are knowledgeable about or that is original) vs a dishonest human (who writes filler content just to get a higher ranking). It would almost need to be able to read the author's mind to figure out their intention. Or, given a text, figure out if what it is saying is original and/or true.
No, they are trying to distinguish between content that is going to make $ for them vs content that is not going to make $. They don't care about truth or utility.
The way I read it, this is aimed at derivative texts that were scraped from other sites. If Google indexes those sites, they can recognize when the content of a newly indexed site is copied from elsewhere.
Most of the blogspam on the Internet today are website filled with product 'reviews' and affiliate links. These aren't derivative of anything, so they won't be downgraded. Neither will the hordes of blogs breathlessly reporting celebrity and business gossip. A search engine won't be able to differentiate between the quality of a TMZ vs say Variety.