I expect that device is a hoax or a situation where even the "inventor" and sellers think it is real due to wishful thinking. If it appeared to work, I expect it would be a placebo effect.
https://en.wikipedia.org/wiki/Magnetic_water_treatment If the power were turned off, but the user thought it was powered, I predict it would work as well. That Wikipedia article does not say hoax as such, but if there was a real effect, a proper study could show that. I expect it to be as effective as a magnetic therapy bracelet. With your posting of the link, I initially suspected you might have been associated with the sellers. I don't think that now.
So what is happening with your hardness test? Could it be that the test tests differently with different temperature water? Maybe try drawing a hot and a cold sample. Let both settle to room temperature. Then test both. If you let the samples sit long enough to equalize the disolved gases (oxygen and carbon dioxide are the ones I am thinking of), that would also remove a possible factor that might affect a test.
Is the water from the hot water tap the same as goes through the radiating surfaces? Usually there is a heat exchanger where the heating water and the potable hot water are separated. If the system was in use for a period before softened water was introduced, the hardness materials could have been deposited. Then the softened water would start picking up the previously deposited materials.
My comments are not based on experience.
EDIT:
http://www.wcponline.com/2016/10/15/water-treatment-enigma-part-1/ makes positive comments on TAC.
The only catalytic device that appears to actually meet its performance claims (to my knowledge) is template-assisted crystallization (TAC) media. This process, which came on the scene in 1998, appears to minimize scaling without requiring regeneration or utilizing ion exchange. TAC utilizes polymer beads, not unlike the ion exchange resin in traditional water softeners. These beads, however, contain microscopic nucleation sites that cause calcium and magnesium crystals to form at the site and ultimately detach from the resin into the water as insoluble particles. These colloidal-sized particles do not attach to surfaces and are carried out with the water. As a result, although TAC does not actually remove hardness, it does minimize scale attachment to surfaces. This process requires no power, chemical addition or backwashing. The life of the resin is typically about three years. It has been thoroughly tested by credible, third-party institutions and has been shown to generally perform as claimed; however, the local water chemistry appears to have an effect on performance.
The last time I tried to find info on independent testing, I was thinking that the testing info offered was not independent. I am not at all a good judge of such stuff. If it is that effective, I would think there would be a fair amount of independent verification.
