I expect it is not nearly as hard of a cut off as described, but since they say nothing about it I can't really evaluate it.
In general, all binders have the attribute that the higher the phosphate concentration, the more binds to the binder, and the lower the concentration, the less binds. So theoretically, a material can bind very little below 0.04 ppm phosphate, but bind more and more as the level rises above that. But I really doubt it binds a lot at 0.05 ppm and none at 0.03 ppm. My pharmaceutical phosphate binders have a similar attribute, binding a lot at physiologic phosphate levels (hundreds of ppm), but essentially none at reef tank levels.
I am very skeptical of their claim that phosphate does not come back off. I just don't believe that doesn't happen under the circumstances where all binders release phosphate: you bind a lot at high phosphate concentration, then expose it to water with lower levels. In that scenario, all of them will release some phosphate.
In order for me to believe that there is a sharp cutoff at 0.04 ppm, I'd want to see what is called a binding isotherm, showing how much phosphate is bound per gram of binder at phosphate concentrations from 0.001 to 0.5 ppm phosphate in the final solutions. I've seen these for hundreds of binders.
