"God is the supreme being."
Translation:Ο Θεός είναι το ανώτατο ον.
Are you suggesting that the Hebrew source from which the Septuagint was translated did not contain the Tetragrammaton? Or are you suggesting that the Greek translation was superior to its Hebrew source text? Regardless, apparently the oldest LXX manuscripts did use it within the Greek text; in some cases using paleo Hebrew letters, others in Hebrew square script and others with it transliterated into Greek letters. The substitutory use of Κύριος / Θεός only appears in later manuscripts. https://en.m.wikipedia.org/wiki/Tetragrammaton