This is almost definitely the problem they're facing although I think that description is a little bit odd. It's missing the operative word: "portals". It's the XDG desktop portals service that is involved here. What you need to ensure is that you have a desktop portal provider set up that provides org.freedesktop.impl.portal.FileChooser. What's kind of neat about the way xdg-desktop-portals is architected, you can pick and choose different implementations for different services. This is especially useful outside of desktop environments where you might need to use e.g. the wlr provider for screenshots and screen capture, but you still want e.g. KDE file dialogs.

It's unfortunate that the documentation for XDG desktop portals (and generally, setting up a complete desktop setup when using compositors like labwc or Sway) is relatively poorly documented. I have my feelings about the pervasiveness of DBus services everywhere but overall I like desktop portals.