Is it?
At the end of the day, whether it's effective or not, Duolingo sells the feeling that you are learning a language to people. Winning a competition with Duolingo means doing better at making people feel like they are learning a language -- the strategy to win against Duolingo probably involves watering down the learning even more, to better sell the feeling.
A good way to think about it is look at some organization that wants to be effective at actually teaching its employees a new language, like the state department:
https://www.state.gov/foreign-service-institute/foreign-lang...
20 hours a week of intensive instruction.
Spanish 30 weeks
Cantonese 88 weeks
Turkish 44 weeks
This is what it actually takes.