That's not true. Memorization is the most efficient way of learning a language, but it often comes in forms which aren't as obvious. For example you can look at reading foreign texts as a kind of repetition for every word you encounter.

It's hard to master and complete a Duolingo course without getting a grip on the language. You'd have to pretty much ignore the "Strengthen your skills" button and never repeat anything.

Even after finishing a course you should use the repetitions for weeks and months to keep refreshing your memory. That's the only way it may stay forever.