All translations have strengths and weaknesses. The LXX is no exception. Caution is warranted with every translation.
Matthew and the early church believed the verse in question in the LXX was translated correctly by the Jewish scholars. Do you agree or disagree with Matthew and the early church? Do you believe, as they did, in the virgin birth of the Messiah?
Perhaps the following will explain my position: -

The LXX English and Greek text of this same verse states this: -

And in Matthew 1:23 it states this in the Greek text with the Strong definitions/meaning of the Hebrew Root words given: -

The consistent message is that Mary was a young woman, a virgin, which defined her character before God.
Now in my answer above I have not answered your question as to what I believe is true with regards to the virgin birth of Christ, but my opinion or belief, is, I believe, not in question.
@Eliyahu613 on his part is probably using the Jewish playbook to counter Christians and as such he may not be stating what he genuinely believes or has questioned whether the provided information in that playbook is actually true.
At present Jews worldwide are still rebelling against God and not repenting of their continual idolatrous worship, even during this present time.
Shalom