Erman is a terrible scholar, for one. The other thing is that, for centuries, the Septuagint was used as the Bible. It wasn’t treated like a translation the way we do. It’s that, for a long time, the Bible was Greek. It had been Hebrew, then it was Greek, then around the 600s or so it was Hebrew again. That later Hebrew Bible is called the Masoretic Text and was the one chosen by Protestants for the Old Testament because, in their thinking, the Hebrew was older than the Greek. But they didn’t really consider the fact that the Masoretic Text is over a thousand years newer than the Septuagint (it being a reconstruction of the Hebrew Bible based on re-translating the Greek with the aid of the Samaritan Pentateuch, etc.). So the Septuagint used the Greek term for “virgin” which is the only reading the gospel writers would have known. The Masoretic Text translates the relevant Isaiah passage with a Hebrew word that means “maiden.” And there’s some argument out there that they did so in opposition to the Christian reading of the passage. There’s a really great book about this entitled When God Spoke Greek.

TL;DR, The “virgin” reading is accurate to the ancient understanding of the Isaiah passage because that was the only one they had at the time. The “maiden” reading is known to us from a Hebrew text that is at least 600 years more recent than the time of Jesus.