That gets my normal GTK theme properly. I found a little more discussion on this here. Nothing very actionable but I did also confirm that my xdg-desktop-portal-gtk is running. It seems like this is supposed to be working, but I have a mostly stock Debian 12.1 KDE install and something seems to be wrong somewhere in the chain. I’ve also tried multiple GTK Flatpaks with the same results.
Edit: Also, I have both my themes folder exposed and the theme installed as a Flatpak via the linked script.