Why include that capacitor at all if it doesn't matter whether it works?
So you can see why it probably didn't matter that this capacitor didn't work: It's only needed for rare occasions. RS-422 is a differential form of RS-232 (https://en.wikipedia.org/wiki/RS-422) so being differential it's fairly robust against changes in load if they affect both wires. And the worst that can happen is you lose a few characters from your external modem.
In addition, electrolytics can probably work when reversed like this, at least a little bit. It's not exactly optimal and they might catch fire(!).
The two RS-422 ports are actually used quite often on these old Macs for printers, modems and apple talk networking. It was the only communication port, as there was no parallel port. They were backwards compatible with RS-232.
So it obviously worked well enough.
The backwards cap was measured to reduce the voltage to about -2.4v.
I suspect that all it did was reduce the maximum range, which started at a massive 1,200 meters for RS-422 (and a good 10m for RS-232)
When I was a demo coder my artist friend would just haphazardly go through all my assembler code and snip random lines out until it stopped working to improve performance.