According to the theory that when using flour from Kitniyos (etc.) one would need to eat the entire Kezayis of actual wheat flour in order to say Birkas Hamazon then it would follow that in Hilchos Pesach 453:7 he would be required to eat the entire Kezayis of actual wheat flour in order to fulfill the obligation of Matzah?
Yet even according to the most strict opinion there if there’s a majority of wheat flour all you need to have is a Kezayis of that mixture!
The sentence בסדר ברכת הנהנין פ”ג ה”ב כתב שכשיש כזית בכא”פ, ברכתו מזונות is a mistake because there he only requires כזית בכא”פ in order to say על המחיה. However, you would need ממשות in order to say מזונות.