These are not “Karen’s allegations”, they are a reference to a Jewish rabbi’s position on the issue. Since the Septuagint and Isaiah (in any form) are both Jewish documents about the details of the Jewish religion, seems reasonable to seek a Jewish viewpoint on them. If you object to the conclusions of Rabbi Singer, please feel free to take them up with him.
Here’s some additional info on the various revisions, etc in the development of the Septuagint
kalvesmaki.com/LXX/
Philo of Alexandria (fl. 1st c CE) confirms that only the Torah was commissioned to be translated, and some modern scholars have concurred, noting a kind of consistency in the translation style of the Greek Penteteuch. Over the course of the three centuries following Ptolemy’s project, however, other books of the Hebrew Scriptures were translated into Greek. It is not altogether clear which book was translated when, and in what locale. It seems that sometimes a Hebrew book was translated more than once, or that a particular Greek translation was revised. In other cases, a work was composed afresh in Greek, yet was included in subsequent collections of the Scriptures. By observing technical terms and translation styles, by comparing the Greek versions to the Dead Sea Scrolls, and by comparing them to Hellenistic literature, scholars are in the process of stitching together an elusive history of the translations that eventually found their way into collections.
*By Philo’s time the seventy-two translators enjoyed a religious cult (Philo, Life of Moses 2.25–44). Pilgrims, both Jews and Gentiles, celebrated a yearly festival on the island where they were purported to have conducted their work. The popularity of the legend of the translation helps to explain why, when we first hear Christians explicitly mention the translation in the mid-second century (SS Justin Martyr and Irenaeus), the entire Old Testament in Greek, whether originally written in Hebrew or not, is credited to the Seventy-two. Thus, from the second century onward, Christians embraced a Septuagint that encompassed a larger body of literature than that found in the Hebrew, and they attributed it all to the work of the seventy-two ancient Jewish translators. *
The information comes from someone who describes himself as follows: “
I am an independent scholar in early Christian studies (Ph.D., Catholic University of America 2006).”