Why include that capacitor at all if it doesn't matter whether it works?
If you look at the traces you can see the capacitor is right next to the power connector, on the -5V rail (which is not used for much, only for the RS422 serial port). The capacitor will be there to smooth the power supply when the machine is just switched on, or there's a sudden load which causes the voltage to "dip" above -5V. Basically it's like a tiny rechargable battery which sits fully charged most of the time, but can supplement the power on demand.
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(!).
Also known as the Madman Muntz theory of Engineering :-)
I never knew there was a name for this :)
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.