That's interesting considering how Mary interacted with the angel at the Annunciation. The atmosphere of that scene sure seemed to have the angel in the higher position.
The angel (meaning a messenger) merely carried a message from God, so in carrying out the message he was speaking on behalf, as an ambassador of God, and as such was in a "higher" position.
But angels are not treated as being higher. There is an instance in the NT where someone kneels before an angel and the angle rebukes him for it. Yet, angles are said to be 'higher' than humans in other parts of the Bible, but they will be judged by humans for some reason.
There is even reference that the angels can see God, iirc, while other references say they can't, so Christian angeology is very "cut and paste." probably as a result of varyius influences that shaped it over the centuries.
The particular hymn is interesting because it mentions the hierarchy of angelic hosts (which means the orthodox laity is aware of it), while other Christian traditions seem to neglect it to a large extent.
Mary's position as the one sitting at the right foot of her Son is part of Marioogy that developed over the centuries in the Undivided Church. As such, she is seen in terms of honor as higher than any angelic creature in the eyes of the Church.