Duolingo is a game, they publish papers studying the addictive properties of their product I comparison to slot machines. They are incentivized heavily to not produce fluent learners. If he's being pushed in another direction to learn, for whatever reason, all the better for him.
Duolingo used to be a very effective gamified language learning system.
Then they decided they cared more about profits than providing a quality product.
Now they are best known for their dark patterns.
Yep, I noticed recently that on their official research portal they stopped publishing anything publicly in I think 2021. The last couple of articles before that were along these lines:
https://research.duolingo.com/papers/yancey.kdd20.pdf
I assume all the research after this point is too revealing to publish.