Have they (and you) been learning language in other ways concurrently with Duolingo, or relying exclusively on Duolingo?
I find it's a useful adjunct to immersion or in-person language learning. Not a complete way to learn language in and of itself (though I question if such a thing exists at all).
It's a good tool but it's not enough to learn a whole new language. As a user it helped me a lot to memorize new words and also to easily understand common sentences and expressions. For speaking and writing skills is much more limited and you would need a teacher and real-world practice.
But I used Duolingo + Anki for all the vocabulary stuff and then I could focus the paid lessons on grammar + writing + speaking to be more effective (and save time & money)
I learned a language living in the country where it is spoken, mainly using DuoLingo for vocabulary and reading comic books for idiomatic usage. I don't get a tonne of practice because of lockdown but DuoLingo plus immersion definitely got me way further than I could have done without it.
I did about a year of Duolingo. I didn't learn the language, but I learned enough that I had a chance. It is hard to learn a language, and Duolingo is a good first step: it gives you a good grasp of the most common words/grammar - enough that you can jump into something that is fun (that is you can understand enough content to do something other than focused study).
There are better ways to learn a language, but there is no method that has every possible language you might want to learn. DuoLingo is a tool in the toolbox, use it and move on. Very few tools will actually go from 0 to fluency in a language.
I don't think he means that there is anything wrong for Stardew Valley, a game, to not be "rewarding" (and thereby rewarding here is defined as something beyond entertaining). But if Duolingo as a learning tool is like that too.. that's a problem.
I find it's a useful adjunct to immersion or in-person language learning. Not a complete way to learn language in and of itself (though I question if such a thing exists at all).