I don't know where they got those numbers. This graph seems more realistic to me [edit: maybe not - see subcomments]:
http://sotagtrends.com/?tags=[ionic-framework,react-native,f...
That is, Flutter is shooting up and overtaking Xamarin but still only half the popularity of React Native. This also roughly correlates with mentions on Google Trends:
On job websites React Native is way ahead - which you would expect given React Native's headstart. For example, www.jobstreet.co.id currently has 43 jobs for "React Native" but only 5 for "Flutter".