What he does do very well is show that the Septuagint was the foundation on which the NT Scripture was built. There are only six verses where the Apostles quote Hebrew over Septuagint. In all other OT references (2/3), the Septuagint version is preferred, or both the Septuagint and MT agree (1/3 minus the six verse mentioned earlier).
I don't see it as an exception or a contradiction
But all Septuagint versions call her a virgin and not a young woman.
That's fine, the Hebrew means essentially "a" virgin also. The young woman is chaste, as the same word is used elsewhere in the Hebrew OT. I'm not sure what else I can add. I don't know which version(s) the Apostles used at the time, but it is true that they quoted from it very often.