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.
Yes, the whole thing is built like bike brake cables, with a spirally metal support core, the goodies up the inside, and then a nylon braid and PVC jacket over the top.
I have issues with the PVC-jacketed cables under the bonnet of my nearly-30-year-old Landrover, where the plasticiser has been baked out of the insulation and they've gone brittle. Worst affected are the wires to the fuel injectors and the lambda sensors, presumably because the former are at the top of the engine and get reflected heat off the bonnet, and the latter because they're near the literally red-hot exhaust downpipes.
That's okay for an old vehicle that you'd expect to repair, though.
I've seen the same problem in three-year-old Toyotas, and that is Just Not On.
The outside strands are kevlar. The fibers are inside the armored core often inside another jacket.
The metal coil will hold the actual fibre itself, yes - after a few more layers of protection. This is what is usually called “armoured” cable and is suitable for suspension and direct in ground. Dunno why he’s using it indoors.
Honestly, this writeup is… weird? Dude doesn’t know you can terminate fibre at home with like $50 of gear?
I had the fucking fox attack a freshly laid 500 meter line, literally the day before I was going to stuff it in conduit and bury it. Didn’t just break the fibre, she (I know this fox, well) chomped it into pieces, hauled on the exposed Kevlar, generally had a party.
Did I despair? Did I launch a baby complete with bathwater into the sun?
No. I bought a cleaver, some alcohol wipes, some stripping pliers and a whole bunch of mechanical terminators.
Needn’t have worried. Repaired it, outdoors, first attempt, in the rain, and have since buried it - no problems five months on.