You are the one who claimed that by rejecting the book with the Assumption of Holy Mary in the title, that he was therefore declaring any teaching of her Assumption a heresy. Unless you have something specific from him where he claims this very thing, then you don't know why and my saying that to you does NOT imply that I do.

Most of the "gospels" and letters that the Church rejected were due to the fact that they were written much later than the Gospels and books that did make it into the NT. The NT was written within 100 years or less of Jesus' Ascension and had the support of oral Tradition.