Infrared are longer wavelengths. Probably too long to be trapped the way they describe. I immediately wondered about microwaves, i.e. radar. But those are way longer waves, so no as well. The reason they can put a mesh screen you can see through in the door of a microwave oven, is because the wavelength of those radio waves (that is really what they are) are so long they can't get through. Finally, black items can radiate heat, you just can't see it. So sure, if it trapped enough visible light, it could be 'converted' to a longer wavelength (infrared) as the fabric tries to shed the extra energy.