It is mistaken to write "There are Greek translations of the Bible that date from the fourth century A.D., several hundred years after the scrolls" because there are earlier examples.

I think that David Plotz meant (or at least I took him to mean) "extant." AFAIK, the earliest extant manuscripts of the Greek translation of the Bible (the Septuagint) date from about 350 CE. Of course the Greek translations were actually accomplished around 150 BCE, but none of those existed before the Qumran discovery. If there are earlier "extant" examples, I'd like to know about them.