how can UPF fabric block UV light, even though visible light can pass through the pores of the material?

pic related is a "buff" brand neck gaiter, made of 100% polyester. the light is through 2 layers of material, so twice what it would normally be. the fabric is not stretched, it's held loosely in one hand. i can wear it over my eyes, and still read text on the screen.

how, then, when visible light can pass through the spaces between threads, is this garment going to block UV light, which has an even smaller wavelength?