These are the last words of God recorded by the last prophet, nothing more for four hundred years, unto John Baptist's ministry began, which John himself knew, and of which he quoted from Isaiah. I believe this is figuratively speaking of John, not of the literal coming of Elijah, which occurs in the midst of, not before, that great and terrible Day of The LORD (Rev. 8-11). With this prophecy from Isaiah, John Baptist, as a prophet, ended the centuries of God's silence (Is. 40:3-5).
Could be but Matthew 17 suggests that Jesus thinks of Elijah as one who is still yet to come despite also
being of the person that was John the Baptist.
Since we haven’t seen the Dreaded Day of the Lord yet, we haven’t yet seen Elijah who would turn the heart of the fathers to the sons and the sons to the fathers. Haven’t seen anything like that yet.