the facts are that origen's hexpla was based on the septuagint which was in general use at the time.
the only ones to say otherwise are protestant denominations with zero supporting evidence. History and Evidence is clear that the Septuagint predates the hexpla by 200+ years
Most of what I have read indicates that only the first five books were well translated from the Hebrew into the Greek before the NT period. Therefore anything that calls itself "Septuagint" would have to match the Hebrew MT in those five books every closely.
Biblical scholars furthermore indicate that the other books that were translated were not done as well as the first five and done by those who didn't really know what they were doing. Translations of these other books may have been attempted at the synagogue level by those with only a limited knowledge of either the Hebrew and or the Greek ---but they never followed through all the way.
It would seem reasonable to me that as the Jews in the NT period were throwing all their Greek translations away, that Christians would pick them up and try to finish what they started. There were probably numerous Greek translations of OT books floating around with numerous variations and different levels of accuracy in different passages.
When people referred to a Greek translation of an OT book, it was called "Septuagint". That word meant "Greek translation of OT". But there was clearly no fixed standard "Septuagint", otherwise Aquila, Symmachus, Theodotian would have not have undertaken their translations.
And if there had been a fixed standard "Septuagint" in Origen's day, it certainly would have been in column one, or two, or three, or four --- but it wasn't there at all. And he used Theodotian's translation to create column five which became his LXX.
Why undertake a great work like the Hexapla if the goal of all that effort, time and money was already in your hand??? Origen's Hexapla is testimony to the fixed nature of the Hebrew text which never changed over time. It also testifies to the fact there was no fixed "Septuagint", just multiple Greek translations that differed from translator to translator, and he was trying to create a single one in his column five which became the OT of Codex B.