They're not trying to teach pure MSA here. They're trying to strike a balance between MSA and spoken language. As they mentioned in an article linked during the course launch:

we’re teaching a less-formal, spoken version of Modern Standard Arabic — not the version that would appear in poetry or formal news broadcasts, but instead the version that would be used once a newscaster stopped reading from their script and started talking to their interviewee. It’s a version of the language that can be used in a formal conversation, but one that also can be used with the widest range of Arabic speakers.

You can read their full article here https://making.duolingo.com/what-makes-arabic-hard-and-why-that-shouldnt-stop-you-from-learning-it