Why is potassium phosphate KH2PO4 in this reaction?
I think that’s just another mistake the OP of the other question made. Potassium phosphate is $\ce{K3PO4}$ and any student using any hydrogenphosphate instead would lose marks if I were marking the exam.

If $\ce{KH2PO4}$ is meant to be added, use the correct term: potassium dihydrogenphosphate.

This nomenclature can be found in the current version of IUPAC’s Red Book, sections IR-8.2 and IR-8.4 and the anions in question are explicitly named in table IR-8.1 therein.