> If the claim is "30min of structured learning with duolingo will get you no closer to learning a language than 30 min of mindlessly scrolling facebook cat videos", I don't believe that's true. And THAT is the correct comparison :)

The correct comparison would include the effect size on language learning of using Duolingo instead of scrolling through cat videos. The effect size appears to be close to zero. And without considering in the analysis the long-term negative effects on the self-perceived ability to learn a language after spending hundreds of hours using Duolingo and being able to simply say "Hola" in Spanish, which may make the use of Duolingo a net negative on lifetime language learning.