They did not "then tell the public" that "the one that convinced them" was this one study. In communicating with the public, they happened to cite #21, because

They did not "then tell the public" that "the one that convinced them" was this one study. In communicating with the public, they happened to cite #21, because explaining all 21 would take too long and be confusing for you, and be distracting. Refuting the specific evidence in #21 does not directly refute the other 20, and does not make the conclusion "Junk Science" as the headline apparently unintentionally implied. And the headline had no business saying "The study," when they have no idea how many studies the CDC internally considered.