You don’t need to run an X server on the headless server. As long as the libraries are compiled in to the client software (the GUI app), it will work. No GUI would need to be installed on the headless server, and the libraries are present in any common Linux distro already (and support would be compiled into a GUI-only app unless it was Wayland-only).
I agree that a GUI-only installer is a bad thing, but the parent was saying they didn’t know how it could be done. “ssh -X” (or -Y) is how.