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!

Old analog tv over coax used to work with the cable poorly seated. You could even disconnect the cable entirely and just point the conductor at the plug and still get a(n admittedly quite noisy) picture as long as the cable end was near enough.

Honestly, before we had municipal fiber, I'm pretty sure our connection was wet string. Rural living does have its drawbacks.

What is the impedance of a gopher's teeth?

The string is dry.

Crosstalk or other interference coupled from the outside into the line. The telco grade twisted pairs do not really behave like twisted pairs (that cancel out the external coupling) at the frequencies used by xDSL.