Memetic polyalloy is not a real substance but is referred to as liquid metal. Obviously the t1000 is not the same thing as the t800 on the molecular level. The t1000 is more like organic metal. Key word "organic". Living metal. You think that's an impossible feat, the t800 has organic skin tissue and anatomically correct. Skynet created living tissue, it's not beyond the realm of possibility that they could make living metal.