Silnylon is siliconized nylon fabric, with the silicon embedded in the fabric. Nylon is flame-resistant.

Waterproofed polyester is probably just regular polyester (no natural water repellency that I've ever heard of, but it doesn't absorb moisture) coated with ScotchGuard or something similar. Some polyesters have built-in fire retardants, but I don't know if all of them do.

On the design of this particular shelter, it doesn't seem conducive to use with a fire/reflector. One might think that a person who carried a shelter would also have firemaking ability.

OTOH, I just looked and the site is based in Tempe, AZ. Shade, rather than heat, might be the biggest need there.

Sue