In the article: "However, one specific requirement in China’s latest policy shift regarding private education is that teachers based in overseas countries are “strictly banned” from teaching to students in China."
China seems to be cracking down on international Edtech services, and these apps are collateral damage. As the article mentions, Duolingo isn't a human-taught service, so banning it feels a bit weird. But I supposed that the purpose of the ban is to reduce western influence in Chinese Education, so the authorities don't really care.