Mh… No.
SteamDeck uses KDE Plasma. So just this push the development of it. Xfce is made with gtk.
I don’t know if most apps are GTK, but indeed it is full of QT, Tk, etc. apps out there. And anyway it does not matter, those runs well on every DE. If you want to make them follow system theme take a look at archwiki. The only issue is QT and GTK have a lot of different dependencies.
The truth is that you have to chase wayland, because it is the future.