No, it is not an astonishment, for it has been done before... The whole bit comes right from the Pharisees; who, in their hubris, thought to trump the tradition of Moses with their own traditions. They claimed the authority to even bisect sentences to draw out the 'true' meaning of Torah, even causing their interpretation to be the full and exact opposite of what the sentence said in full.
It is the very definition of wickedness: Twisting the scriptures to obtain a desired result. It is how one gets to subject matter like the OP.
Throwing out part of the Old Testament is an explicitly anti-Christ, anti-Christian, Pharisee dictate.
Neither Christ or the Apostles ever once in any way implied any portion of the Septuagint shouldn't be accepted as the inspired Word of God.
If that's the "whole bit" you mean you've hit upon the easiest way to identify modern day Pharisees, they obey anti-Christ, anti-Christian, Pharisees and throw out portions of His Holy Word to avoid the Truth contained in those portions of Scripture.
Pharisees didn't like how clearly those portions of Scripture laid out exactly what Christ and the Apostles taught and early Christians practiced.
Luther knew The One, Holy Catholic and Apostolic Church Jesus Christ Himself founded preached, taught, and practiced exactly the same way the early Christians did and so he resorted to the same elimination of Scripture as a means to hide his evasions of the Truth.
People who prefer Pharisees and Luther to Christ will one day hear, "I never knew you" from the very same Jesus Christ they imply is a liar by pretending the Holy Spirit cannot and did not protect His Holy Word from the inclusion of error.
We have folks trying to do this very thing on FR these days!