Correct, the default core material for the GT2 belts used is fiberglass.
There has since been some experimentation with belts which use alternative core materials:
Steel:
Kevlar:
(you can search for more)
Carbide 3D has begun to switch to steel core belts for machines— I got a set for the SO3 I keep at my mother-in-laws for when I need to do tech support while visiting but haven’t installed them yet (something for the next visit).
I didn’t ask for a set for my XL since the belts on it are still in good shape, and I paid rather a lot for them from SDP/SI — they are actually Gates-branded, I believe made in the U.S. (and I’m trying to keep my XL stock/entry-level, while the SO3 is getting all the cool upgrades, starting with an aluminum table, and eventually heavy duty eccentrics and HDZ) — while I can understand the appeal of the alternate materials, I want to get my money’s worth out of these belts before I replace them.