Where's the WIMP GUI?

Last night I installed the flatpak of Retroarch. First, let me say thanks to the devs for doing that, the Fedora packages have been sorely slow in updating.

It’s the latest version 1.7.3 on Linux. The latest blog post on Libretro’s site says that on Linux I should be able to switch to the WIMP GUI by pressing F5, but that didn’t work. Do I have to install something else? Or in case KDE is intercepting that call, is there a way to do it from the PS3-like GUI?

I believe the flatpak packager ran into some difficulty packaging the WIMP GUI.

I do the packaging for the snap package and was able to get it working there, but it was a hassle, so I can imagine there are similar hassles involved in the flatpak.