While there is fiberglass that is present within the cover, it is not on the outside portion of the cover. It is only on the inside fire retardant (FR) portion of the cover. The glass is a portion of the FR thread and is completely encased within other thread components.
To get a sense of what that means, imagine a pool noodle. The central empty space of the pool noodle is where the glass would be in the FR thread. The other thread components would be the outside 'shell' of the pool noodle. Glass plays a crucial role in the fire retardant properties as it will not combust, it helps to maintain the integrity of the fire barrier during the test of the 16 CFR Part 1633 Open Flame Standard.