The study supports some popular beliefs about tenure-track professors, but people shouldn't be too quick too generalize. This study was very limited.
From the Atlantic article:
"Now time for a few disclaimers, some from the paper, some my own. As the authors note, this paper only looks at freshmen. Tenured professors might very well might do better in advanced junior and senior-level courses where they can incorporate their own research and special expertise into their curriculum and have a chance to work with students who've accumulated a bit more specialized knowledge. Also: Northwestern is a tony private university that attracts highly qualified faculty to work as adjuncts and non-tenured instructors. Who knows if these results would hold up at a typical state university. "
What holds for Northwestern freshmen may not hold for other populations. Such cautions are being ignored by a media (and a few intelligent commentators who should know better) too eager to confirm preconceptions.