Of course it works. It is a normal GTK program with some Gnome specific dependencies (libsoup). There is no reason why it would not work on XFCE. You need to ask the users that don’t get it to work what they did wrong.
The answer to the original question “why this did not work somewhere”: because somebody somewhere had some OS that was broken, not updated, missing some dependencies or having some conflicting packages or maybe stumbled on some rare bug.
If a package works equally well for all desktops (I am not assuming this is the case since only two of them came up expressly for discussion), then I would think the qualifier “for the GNOME desktop” is at least misleading and maybe downright wrong.
If this is unclear, just analogize from “tennis rackets for women” for rackets that work for anybody.
For all desktops might be a stretch. For a desktop mainly using the GTK toolkit would be a better fit. But in the end it does not really depend on the Desktop, and more on the additional libraries that can be installed on the system.
All programs develop by the Gnome Projekt are “for the GNOME desktop”. It is intended to be used with the Gnome desktop by its developer. If there is a problem with it on different environments a bug report might be closed since it is intended for Gnome. But this does not mean it does not work at all on different desktops.
Thank you for giving me an idea of the complexities that might be involved in making a package a good or bad fit for an environment and confirming my first suspicion that I (not knowing about them) had no reason to assume anything was supposed to work.
If ever I run into a mismatch between a package’s (expressly) named desktop and my own desktop I will be sure to ask again this valid question.