Hmmm... a bit of a con job, methinks. Anyone who rides in the rain will know:
(a) Splash from the front tyres gets shoes, socks and lower legs wet; a fender and flap will mitigate this, but there will always be spray thrown forwards beyond the fender, then backwards because of the slipstream once any surface water gathers on the road and the bike's speed increases.
(b) In any sort of rain, water runs downwards -- that is, towards the lower leg, socks and shoes.
(c) In really heavy rainfall, every part of an unprotected leg gets wet. Even in lighter rainfall, the water will run down these and seep through the back part of these chaps.
(d) Cloth acts as a wick to draw water in through openings at the ankles (and wrists) and wearing these will delay the inevitable by... oh, I'd say... five minutes.
It is noteworthy that the things are shown on an equestrian rider who has knee-length boots.
I do recognise, however, that after suffering on several rainy 1200 randonnees, that thigh warmth in wet weather is essential to keeping the rest of the body warm and other leg muscles functioning, because of the volume of blood that traverses those two bodyparts.
Plastic shopping bags, suitably split and shoved underneath leggings were the temporary fix in the absence of suitable waterproof trousers. At other times, when the temps haven't been so cold, I have worn polypropylene leg-warmers and just got wet -- and was comfortable.
In France, Decathlon carries cycling spats made of a similar material as their waterproof trousers. The spats cover the shoe (and a lot of the sole) and come up part calf-height on the leg, with a Velcro closure. They are ostensibly designed for wear with ordinary clothes to keep shoes and trousers drier on short utility trips; combined with waterproof trousers, I think they would be too hot and sweaty.
My major concern in the rain in cold temps will always finding ways to minimise exposure of my shoes to the rain. There is a model of Shimano MTB shoes with a Gore-Tex lining available here in Australia for a smidge under $200 that I might be tempted to acquire come next winter.