That’s a bad description of it, though. There exist flash card apps, such as Anki and Memrise, and Duolingo has many other features. Grammar, reading, speaking, those are all built-in.

Of course some people prefer to learn grammar using the old-fashioned “grammar translation” method, and you are free to do so if you want to, but on average the speed of that method is slow.

Certainly it’s a good idea to have at least two study materials, which could include Duolingo, or not, according to personal preference.