Because the text-only ads look much like search results, it is arguable they never needed to cross the line. Having the ad urls appear at the top of the search

Because the text-only ads look much like search results, it is arguable they never needed to cross the line. Having the ad urls appear at the top of the search results page and be relevant to the search query was enough to get people to follow them. However, I think it is indisputable that the "insidious bias" referred to in the paper has existed for many years, where parties pay to get listed. Of course, they might not pay Google. Consider "Parasite SEO" which was just on HN front page.