” I can only guess that he was machmir for “bliyos” of kitniyos.”
Being “machmir” has to have some halachic basis. I i told you I tie my hand behind my back all of PEsach. Why? To be machmi, you woudl rightly question said “chumra”
Kitniyos is batal berov, the chicken soup was muttar to eat on Pesach (unless it was Rov corn, which is hard to imagine) and obviously the pot was muttar too. The Chavos daas says it can even be used ben yomo. but certainly the pot is fine after meis leis.
Obviously we dont lechatchila, add kitniyos to food even if will be batul, and arguably the woman in question had other soup available so even once added it was easy to eat other food. But that doesnt change the halacha that the soup and certainly the pot where muttar