I certainly believe that. I think my rocks are holding a lot of phosphate too. In addition to carbon dosing, which really only seems to reduce NO3, I occasionally dose Phosphate Rx. It always rebounds, but the peaks seem to be coming down. My peaks now seem to be around 1 a couple weeks after dosing. I understand I need to have some phosphates for the carbon dosing to lower nitrates, so I have not been very aggressive with the phosphates.K. you will likely find that the aragonite holds and releases lots of phosphate even after your water P level is way down.
It will slowly come to equilibrium, but there's a lot of bound phosphate between here and there.