The pAg of silver in silver nitrate solution being meaningless means that it has little to do with making an emulsion. Certainly, the silver in a silver nitrate solution has a certain concentration, and thus it does have a pAg.

When you mix your silver nitrate solution with halide in the kettle, the pAg is going to radically change, i.e. get much higher, as the solution will not be able to hold as much silver in solution as long as there is an excess of halide in the solution.