If one wire in the pair is still connected, it's not really working because of capacitance (which is ~nil at 5mm), it's working because diff pairs are amazing.

If one wire in the pair is still connected, it's not really working because of capacitance (which is ~nil at 5mm), it's working because diff pairs are amazing. The remaining signal level on the single wire was probably enough to keep the receiver happy (at ~10kBaud in the Reddit post, versus the design ~100MBaud), and so, you win. Any time you can spare the pins & wires to go differential, and have the slightest hunch you might need it, just do it. Diff pairs work!