I suppose the answer will be that one is allowed to sell a product which performs a function even though it does not perform that function to some certification requirement which could be applied to that function.
This is just as well, otherwise everything down to the door handles would need certification. There are enough blatent job creation schemes already in this business.
I fly with a TKS de-iced prop, which I can tell you is highly effective in terms of extending mission capability in typical European IFR scenarios, but no way could it be certified for anything whatsoever. And that is just as well because if it could be certified it would cost 5x the price.
Could also be that the manufacturer decided to not bother applying for certification.
The phrase "known ice" applies to FAA registered planes. It doesn't apply to G-reg for example; the system there is different. Maybe somebody can dig out an online reference to the exact requirements to get certification for various degrees of icing.
The whole subject is also wrapped up with legalities concerning whether you knew there would be ice before departure, and there is much debate on the subject especially in the USA where the law has been enforced in certain cases. I don't think there has been any enforcement in the UK. For example, in the USA, a PIREP of icing constitutes "known ice" but of course there are no PIREPs outside the USA (that I know of); anything you tell ATC might as well go down a black hole, unless another pilot just happens to overhear it.