The people who claim this probably did not have access to the technical data of 5th-generation fighters, and thus their statements are more of a marketing gimmick rather than honest comparisons. We've seen similar statements made in light of the Gripen (which Saab claims can defeat a Su-35), Eurofighter (again, the Su-35), and finally the Su-35 (which Sukhoi claims can defeat the Typhoon). By the same token, the Coca-Cola company marketed their drink as a healthy alternative to other beverages, which they later backed up with their own iteration of so-called "evidence".
Fair enough; count me as a skeptic until you find an analysis that is grounded in tangible data rather than academic ring-tossing.