All of this context to say that not once has anyone using Duolingo been able to "test out" of the first ("101") class that they teach. Duolingo self-learners come in with a very unequal mix of vocabulary and... not much else. Unable to use declension properly [0], unaware of most rules around gender, verb tenses, etc.
I'm sure (and I should look it up) that there have been academic papers written on these quite different methods/approaches: gamified learning vs "academic" learning, immersion by moving to a country, etc.
But in my parents' experience of teaching (which spans ~40 yrs), Duolingo students pretty much all became disappointed in the app: these students thought that they had developed skills when it turns out they mostly got addicted to a game that overpromised useful learning over entertainment.
---
Imho, the ugly truth is that language learning as an adult is deeply hard and requires a tremendous amount of effort and "tricks" to keep yourself motivated. People who watch native media with subtitles, play with AI apps (such as the YC backed https://www.issen.com/ which is quite nice), take a mix of "classic" classes, spend time in a country where the language is spoken and force themselves into situations where they "have" to speak, etc. all do much better. But it's a ton of effort.