Being Flesh and Blood and Dying does not nullify the Assumption.
OK, I think its more important to find what Cyril believed vs a forgery claiming to be from him.
We don't know for sure if it's a forgery,much of it seems in line with Blessed Cyril,but it just cannot be authenticated.
If you want to be really technical we can say the same thing about the Gospels-they did not come with a single autograph to authenticate them as positively coming from them.We accept them on faith, based upon witness of the Early Church Fathers
Traditions of the Apostles came first, the Scriptures came next. God gave His Gospel orally first. The Apostles gave it to others orally first. The Scriptures didn't come until AT LEAST ten years later, if we believe that an Aramaic version of Matthew was written in the early 40's. Thus, the first ten years at least saw Christianity spread without any Gospel writings, any Epistles, etc. Later, when these same men of God wrote letters and the narratives of the Gospels, they naturally taught the SAME thing that they taught orally earlier to others. Thus, the oral teachings preceded the written ones, and the written ones did not overturn the oral ones. Nor does it say anywhere that oral teachings are encapsulated completely within the Scriptures. This is a Protestant assumption that is proven incorrect based on the writings of the first Christians.