The FileChooser portal will never ask for directories, it's currently not possible to expose a directory to the sandbox environment. If you look at [1], you can see it always uses native file dialog in case of directories, which means native file dialog will be opened inside the sandbox, not through the portal.

[1] - https://code.woboq.org/qt5/qtbase/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportalfiledialog.cpp.html#_ZN27QXdgDesktopPortalFileDialog4execEv

This is for non-sandboxed applications that want to use KDE file dialog.

If this is meant to be used by Firefox, then it won't work either, even when you use GTK_USE_PORTAL=1.

What would be the usecase here? I can think of Firefox only, but as I mentioned, it won't work with Firefox, because it behaves exactly the same way as Qt, which means a dialog inside sandbox is opened when requesting to open a folder.