Geany also works on Windows and Macos and on many other DEs which don't seem to be listed as having backends [here](https://github.com/flatpak/xdg-desktop-portal). Also not sure if Windows and Macos do dbus even.
Therefore supporting portals would require Geany to have two builds, one using portals and one using GTK with the associated maintenance and support.
GTK4 (if Geany ever gets there) has a [native](https://docs.gtk.org/gtk4/class.FileChooserNative.html) file chooser for Windows and Macos, but not Qt (can't think why not ;-)