I read through that, and have before, but it works against proof that the apostles taught the assumption of Mary. In fact it indicates that it was NOT taught by the apostles but that the belief grew over time. Basically it’s an admission that it’s something the apostles did NOT teach. Therefore those that teach it are to be considered accursed according to Paul.
What is says in the opening 1-20 stanzas is that the belief in the Assumption is detected in the early liturgical practices and is consistent with the foundational Christian belief in the resurrection of Jesus and eventually all the saints in the body. Further, the Fathers "did not draw their teaching from the feast itself as from a primary source, but rather they spoke of this doctrine as something already known and accepted by Christ's faithful." It is therefore an ancient belief. The petitions to recognize that formally increased through modern history, so the Apostolic See, acting with the same authority as that of the Holy Apostles, recognized that as a dogma of faith.