I have a hard time taking this number seriously, that sounds grossly exaggerated.
Are there parts of the world where people actually use Flutter? Even their showcase is pretty light and a bit deceptive.
If you compare it to React Native there probably are more Flutter apps published but if you drill down into the top 1000 on each app store you would be lucky to find more than a few in most countries using Flutter whereas React Native definitely makes up more than 10%, maybe as high as 30% of non-game apps. I know on my phone I have zero Flutter based apps installed and almost 20% use React Native in some way.
Source: I'm an app developer so I keep an eye on these things.