I don’t share the author’s inerrantist polemic, but I believe the Septuagint should be authoritative for all Christians when it comes to the Old Testament.
Eastern Catholic and Eastern Orthodox Christians still consider the Septuagint authoritative.
Your argument is compelling.
If the Christian world between A.D. 30 and A.D. 400 was relying first and foremost on the LXX, what right have we to toss that overboard just because it doesn’t fit our preconceived notion of what the Bible *should* look like? The Masoretic text is certainly valuable and useful. But did the earliest Christians make use of it? And if not, why not? Maybe they knew something we don’t.
I also disagree with the author’s anti-inerrantist position, but I think he has exposed a big weakness in trying to base Christianity from a *book* rather than a living community of Apostolic faith. The *book* was the product of the community, not the other way around.