A question about length vs electromagnetic interference relation
Because the long wire/trace acts as an antenna. Ideally your antenna length should be equal to 1.25 of the wavelength of the signal but also half/quarter/one-eight works.
https://en.wikipedia.org/wiki/Dipole_antenna
That's the simplest antenna type along with monopole but if you get fancy, much smaller antennas can be made but require non-trivial design process.
EDIT
With regards to the OP edit -
Twisting wires makes sure the noise in the differential pair is common mode, which will be rejected by the differential receiver. This happens e.g. in ethernet wiring since all of the unrelated wires are bunched up in close proximit to each other. By twisting you make sure both wires get exposed to the same amount of EMI field from nearby wires, or least close enough for reasonable frequencies. In a flat cable one wire would get a lot more than the other. Unless you use G-S-S-G configuration which wastes 1/3rd of your ribbon cable.
The second reason for twisting is to create controlled impedance over the cable length which is important for high-speed data transmission. You can absolutely do this with GND + single ended signal but it's not as good as proper shielding i.e. coax.
On a twisted pair the differential signal EMI gets negated by the equal and opposite signals at close proximity. This would work for flat cable too of course. If you twist two single-ended signal wires together, you maximize cross-talk.
Differential signaling does not actually require paired wires, high speed buses can carry the pairs in separate micro-coax wires.