I know that is the theory of many secular theologians but the scriptures themselves are unclear. There is no indication that Isaiah or any other of that era had a son named 'Immanuel' translated "God with us" or "God is with us" from the ancient Hebrew. It is unreasonable to think that if such a miracle had occurred in the 7th or 8th Century BC that a lot more would have been written about it.Except...Isaiah was not referencing Mary, but a young woman (his daughter) who was married to King Hezekiah and who would bear the king's child.
There are only two Christmas narratives in the New Testament and it is interesting that Matthew focused on the virgin birth but Luke did not mention that Mary was a virgin. Luke did say that Mary was with child when she and Joseph went to Bethlehem with no indication it was Joseph's child.
It will be interesting to discuss these things and get the whole story when we get to Heaven. I like to think such discussions will be possible leaning on Paul's comment: "Now we see through a glass darkly but then face to face. . ."
For someone as curious as I am about these things, it is a delightful anticipation.