My understanding is that the aluminized mylar blankets do indeed reflect radiant heat. Unfortunately, you lose most of your heat through conduction and convection where wool and fleece are more effective. If you are in a vacuum (i.e., space) radiant heat lose is far more significant.

Those blankets are oversold. They do keep the wind and rain out, and they have a place in a survival pack for help in concocting a survival style hideyhole, although there are better products for that nowadays (Heetsheets) IMHO.

I use mine for tent footprints. Very lightweight and effective, although a bit fragile.