Yes, I agree that He was born from a Virgin as a sign to show something different. But why is that? Why did Isaiah foretold that such a thing would happen? What Maximus says is not in the Bible by words, but is in the Bible by it's spirit (Spirit). It seems to me that it is unappropriate to say that Jesus had to be born by a Virgin, because Isaiah said so. He did say it in Spirit, nonetheless, but why it was God's plan for Christ to be born like that? Maximus is asking this question, and i think that he gives a satisfying answer to it.
So, Maximus tries to give us a deeper understanding of the reason behind it, and, as a great systematician he is, he does give us an answer that is systematic, holystic in its relation to whole of Christian theology.