I think they're mostly wrong. I think we put too much stock into the default UI toolkits on each platform since this really isn't much different than how GTK and QT work. They have their own rendering engines and have to draw all their widgets from scratch just like this. The only difference is that GTK and QT are blessed.

I think there's a case to be made for sticking to the defaults for consistency's sake but Flutter is just as native as any other UI toolkit on Linux.