This is absolutely true but also applies in reverse. There’s many people who can speak languages conversationally but if they have to read in that language they become lost because all their experience is in conversation. If you’re very unlucky, in some languages (Arabic, Chinese, English) the written language has little resemblance to the spoken language, most languages are more forgiving.
Speaking/listening and reading/writing have overlap but if you want to master a language you also have to be doing exercises with text and reading text. After you reach a certain level of proficiency you need to start reading books (start with children’s books)
As far as Duolingo goes I don’t think it’s particularly good, except as a 101 introduction to a language.
In my experience living abroad in a country where English is well spoken, text is practically far more important. You can always work an understanding out with a human, but you can't ask a signpost to repeat itself or a letter to explain something in English.
Speaking/listening and reading/writing have overlap but if you want to master a language you also have to be doing exercises with text and reading text. After you reach a certain level of proficiency you need to start reading books (start with children’s books)
As far as Duolingo goes I don’t think it’s particularly good, except as a 101 introduction to a language.