That is true (that Flutter builds natively on all platforms it supports, and controls every pixel on the screen, unlike React Native and similar).

But I suspect these people were trying the web version of this (pretty neat, IMO) app.

Flutter does build for the web but then it does (of course) use web technologies to deliver the UI. To get the native experience on iOS, you'd have to download the iOS version from the App Store or something like that.