When filtered to "cross platform mobile app frameworks" anyway, which is a huge reduction in scope - 1/3rd of respondents in that study in fact.
So 46% of 33% of mobile app developers that aren't building mobile websites use Flutter. That's not bad at all, but hardly supports a claim of being the "leading mobile app development framework" as you're down to ~15% of mobile app developers using it. And only then since mobile web is being excluded, who knows what it'd be if that was included.
To add to this, perhaps anecdotal but I've noticed Flutter to be in that specific area of people who like it and use it feel like they're underdogs, fighting in the war to make it the next big thing. It's not the default way to write an app on either Android nor iOS, so they're vocal about spreading the word and getting/keeping momentum.
People who use Swift/UIKit to make apps may like it, but it's also the default way to make an iOS app, so they don't feel the need to fight a war. That language was handed down from above as the winner of iOS development. Same for Java/Kotlin/native UI libraries on Android.
Well assuming Swift and Kotlin split the remaining half 50/50 (I think this is reasonable as most major apps are on both platforms and it is unlikely to use Swit for iOS but flutter for Android), they're probably only 25% each.
Unless you want to count them both as Native at 50%?
So 46% of 33% of mobile app developers that aren't building mobile websites use Flutter. That's not bad at all, but hardly supports a claim of being the "leading mobile app development framework" as you're down to ~15% of mobile app developers using it. And only then since mobile web is being excluded, who knows what it'd be if that was included.