Almost 3 years with no answer. Until today.

[Randy Holmes-Farley Verified Community Expert]

Understood. So it's the Phsyical Structure (pores in rocks, etc) that slows things down (Both ways. IN[Built up] and OUT[Leaching] )
This probably applies not only to Phosphates. Silicon, Alluminum....Possibly Nitrates....However with Nitrates we have Denitrifying Bacterial in those tiny Rock Pores, and they consume it (What makes Quality Porous Rock & Media Great).

So I wonder. I have Products like Marine Pure Bio Media and Seachem Matrix in my System. They probably have similar properties to Live Rock for holding and releasing Phosphate.

The degree of phosphate binding depends strongly on the mineral composition at the surface, and I don't think Seachem or MarinePure says what the exact mineral composition is. I would expect both companies to tout phosphate binding if it was substantial (even though of limited utility in that application).