All that is true, except the last point about GTK. Most developers that want their apps running everywhere picks Qt. GTK is just dead in this game and any developer picking GTK is restraining theirselves to GNU/Linux. Don't use GTK
The way I see it, using GTK reminds me of "native" Mac apps (like the ones written in Swift), but on Linux. GTK sure looks a hell of a lot better than Qt on a GNOME desktop: more modern, polished, and clean. So that's why I'd imagine some people choose GTK.
Ironically GTK apps look better on a QT based desktop than vice versa because KDE/QT devs unlike GTK devs actually make an effort on that front.
You can with minimal effort theme both similarly by installing a theme and icon theme that has versions for both gtk qt but your gnome desktop wont provide a built in gui to configure such, again their choice, their limitation not a limitation of QT. A given app QT app isn't going to pull in a particular theme because that would be the tail wagging the dog.
