Bear in mind X11 is pretty old, and these are some of the oldest libraries in the whole X constellation. That isn't an "excuse", but it's a reason.
However, as the email states, this only gets you the same access your user already had on the remote system, unless it's a setuid program. The canonical example and only one I can think of off the top of my head is xscreensaver or xlock. There are now GUI versions of su/sudo that would also be targets, but I don't think variants of these were used back when this topology was common.
X is (very) roughly the size of GCC. It's massive and it's nearly entirely C, with a few modern Python scripts to generate some of the more onerous tables. There are many old libraries, and they are horrifying. Eldritch, cyclopean, etc.