Looking at the pictures it looks like the fiber itself might be inside of that spiral metal conduit in the middle and the outside is just abrasion protection. There are way too many strands for that outside bit to be the fiber. It's obviously bad that the outside plastic disintegrated, but it looks like the buried cables might be fine.

I have a similar problem on my car where the 12v wiring is disintegrating like this because the manufacturer tried to switch to a more environmentally friendly wiring. Now the wire jackets turn to dust at the slightest touch or if they vibrate too much. I'm forever tracking down intermittent shorts in the wiring harness.