So here's the deal. In the hobby theres a million ways to skin a cat. so The lfs was not far off.
Nitrate and Phosphate are nutrients. Youll hear a lot about that. Nitrate is dissolved organics and floats around in the water. Phosphate(or the kind we refer to in reefing) binds to rock and sand. Over time it builds up on the rock and sand. so old sand and rock can have high levels of Po bound to it. There are many ways to remove it.
The Nopox beefs up the bacteria in the tank to eat it bascily. BUT theres no bacteria or bacterial cycle in your tank to do that right now.
The rowaphos contains GFO Granular ferric oxide polymer . It binds the Po little by little from the water then from the surface of the rock.
OR can you dip it in an acid bath and burn the outer layer of the rock off.
Or you add a chemical called Lantham chloride. Ive never had to use it, I dont know all of the effects esp in a newer tank*probably harmless in my experiences here. but I would not recommend it to a new aquarist.
so when you have GFO in the system its removing the po from the water so making your test inaccurate. Some like to know the true number but I dont think it will matter to you except to know that it is getting lower.
In my Opinion. The high phosphate wont affect the cycle. In fact high phosphate is highly contested subject right now in coral science period. But as you just starting the tank it wouldnt hurt to treat the Po problem.
EDIT Because its Old used rock and leaching is a concern.
Lets go back to some of the choices. You should make one.

I would ditch the RX. I dont know the Po removal rate of Roaphos verses other GFO. But what it will do is slowly lower the Po over the next several weeks. In a week or so I would change out the GFO for new(rinse well) you should slowly see po numbers decline. Once the number get to reasonable level, pull the GFO and wait a day or two test again and then decide if you need to keep using it.
My self I dont use it, my numbers are acceptable. I dont worry much about Po any more, but, your starting a new tank so while you have time you might as well take care of it.