I can only disagree. Flutter is primarily a UI framework. The "functionality" of a mobile application requires access to core features of the mobile platform (e.g. accessing sensors, bluetooth, nfc, contacts, camera, ...). The platform feature access is made available via plugins also when working with flutter. This is the same for RN, Cordova, Xamarin etc. There is no SDK in flutter, Android is the SDK. Flutter has no direct access to Android SDK APIs...
They're changing their stance on that gradually, just recently they started tracking their options for natively implementing multithreading in the background using dart(the current advice is to use platform specific plugins which isn't robust for services that run without a visible app). You're also wrong about xamarin- you do have direct access to android sdks(if you know enough C# to correctly translate the API usage into Xamarin android - which is a bit of a headache admittedly)
i really like the flutter dev community btw, very agile and open to newer ideas for plugins. havent seen this in react(fb controls all and devs usually just talk about js patterns all the time) or xamarin(community is nonexistent at this point, even questions barely get answered unless you post an interesting one on the gitter monitored by core xamarin devs)
The "available plugins" are not the right way to compare old frameworks and new ones. Of course a new framework like Flutter will have fewer.
But also not everybody needs to use "available plugins", you can do a hell of a lot with just the SDK.