This is a development of the standard rain cape used before the days of rain jackets and pants. It's nice that a hood has been added, but, I suspect it would not be compatible with a helmet. The old ones had thumb loops inside to hold the cape in place over the handlebars. The cape would be carried rolled up behind the saddle, usually with the spare tub in the middle.
They clain that your lower parts will not get wet. Your own forward motion is enough for your pants to get wet. We always used "leggings" that came up mid thigh and attached to your belt. The whole ensemble, made of yellow oil-skin fabric guaranteed one would become nice and sweaty!