Here's Xiaomi's Official statement on this: Our product manager of MIUI shared a screenshot on Weibo recently. The main purpose is to collect feedback from the users about their thoughts about MIUI 9's upcoming design. He clearly mentioned that the screenshot attached is one of the themes made by MIUI. However, some tech sites misreport it as leaked screenshot of MIUI 9
Additionally, Xiaomi has also revealed the launch date of the MIUI 9. According to the reports, MIUI 9 is indeed coming soon and likely to arrive before 16 August 2017.
Earlier reports says that Xiaomi has already started testing the MIUI 9 based on Android 7.0 Nougat. The new UI will reportedly come with Split-screen and other amazing features. That said, Don't get spun by Internet rumors.