I feel certain the phlegm comes from my throat. The phlegm,itself has a different consistency and even taste than nasal mucus. I happened to see my doctor when having a particularly bad day. After examining my throat with a lighted tube that went up my nose and down into my throat, he agreed that it was coming from my throat.
I think the symptoms from post Covid are just different from the usual conditions of the throat and nose from allergies, strep, colds,etc..