It has a high cation exchange capacity (CEC), like many other good substrates out there.
The plants can still use water column nutrients too, especially if dosing daily like many of us do. But substrates with high CEC can also remove some nutrients from the water and store them. The plants can then use them via their roots, that is generally a good thing - like spoiling the plants through feeding their leaves and roots.
Tropica also state that their substrate has no impact on pH or hardness, so the CEC can't be "too high", otherwise we'd see a pH and hardness crash, like with ADA Aqua Soil.
Hopefully JamesC, our resident chemist, will chime in.