The fact that John writes this down indicates that he observed the angel’s action. We in turn see the action also in the eye of our mind. The Holy Spirit wanted John (and the church of all ages) to see this action because of what it symbolizes. The angel himself provides the Lord’s explanation.
21 Then a mighty angel took up a stone like a great millstone and threw it into the sea, saying, “So will Babylon the great city be thrown down with violence, and will be found no more;