Hi Mark in Texas,
The “rayon fibers” are the fire barrier that is necessary to pass the 1632 and 1633 fire regulations which are a legal requirement for all mattresses (see post #4 here).
There are also differences between the design and functions of a mattress protector (see post #89 here), a mattress encasement (see post #2 here), and a mattress pad (see post #10 here) from a mattress cover (or tick as it’s known in the industry). Mattress protectors (like the one you linked) and encasements are mainly designed to protect a mattress that already has a cover or tick and have as little effect as possible on the feel of the mattress. Mattress pads are usually meant to fine tune the feel and performance of the mattress although some also function to protect it as well. Mattress covers or ticks will tend to be more durable and are meant to be part of the mattress itself and enclose all the materials and components to help it keep its shape as well as being part of the feel and performance of the mattress relative to its design goals.
Phoenix