Hi Stehno. The point is that the cable matrix is well-shielded from outside influence, as you surmise. Re self-noise, it's thought that by using the very best (practical) dielectric (all-teflon), phase, echo, and hysteresis-like distortions are minimized too. Here in Boston we have a very stiff power grid, and perhaps my dedicated 83802 lines are unusually quiet without further enhancement. It's pretty clear that the noisefloor dropped after I introduced these lines and PCs vs the old Romex and other PCs (stock, Absolute Chinese, Belden 19364, Harm tech ProAcC11, and VD P3) I had on hand. No cable can FILTER out noise, of course.
But sly devil's advocacy can be fun, eh? Cheers.