The Codex Sinaiticus is a Christian codex of the Christian canon written by Gentiles, whereas the Septuagint is made up of Jewish scrolls written by Jews.
The Codex Sinaiticus comes from around 300 CE, but the Septuagint from around 132 BCE.
The Codex Sinaiticus was written in leafs, sown into signatures, an invention of the Gentiles, whereas the codex had not been invented when the Jewish translators of the Septuagint did their work (which is why the LXX is on scrolls).
The Codex Sinaitcus contains the entire Christian canon of books, and it was composed by Christians some 300 years after the Temple fell. The Septuagint is a Jewish translation of the Tanakh into Greek of the Second Temple era, almost 200 years before the birth of Christ.
The Greek translation of the Old Testament in the Codex Sinaticus is a copy of a late tradition of the Septuagint, whereas the Septuagint is not a copy of anything but an original translation.
You claim here that "I have nowhere said PRylands 458 is Christian. It is from the second century BCE so it cannot be Christian." But prior to this, in an earlier post you wrote:
The earliest copies of the LXX used various forms of YHWH or the Greek transliteration IAW. (There are about 7 such examples) None of the Jewish fragments that survive show KYRIOS instead of the divine name. The earliest Christian copies using KYRIOS date no earlier than late second century AD. The fragment of Genesis that leaves spaces for either YHWH or KYRIOS is quite late, from the third century, and probably Christian. It is probably indicative of the transition from using YHWH to KYRIOS in Christian practice.
You write that “the earliest copies of the LXX used various forms of YHWH” though you now agree that the earliest example in the Rylands fragment does not. You also wrote that there are “Jewish fragments” of the LXX that have survived in contrast to a “fragment of Genesis” of the LXX that “is quite late, from the third century, and probably Christian.”
From this I gathered you were saying that the PR 458 is the fragment “that leaves spaces...from the third century, probably Christian.” My mistake if I didn’t understand you, but the only copy of the Septuagint “that leaves spaces” for YHWH is PR 458. PR 458 contains only portions of Deuteronomy, not Genesis, and originates from about 200 years before the birth of Christ.
You added in a later post: “The text is not extant in the places where the divine name would appear.” But as I demonstrated, PR 458 consists only of Deuteronomy sections, and the Divine Name is supposed to appear several times at Deuteronomy 28:17-19 and 27:15 and 28:2, where PR 458 only has spaces instead.
So I will claim I am making the mistake in understanding you. You wrote these things, but obviously you have different meanings behind your words I do not see. You must be talking about something else and thus we are comparing your apples to my oranges.
But your claims about what is the LXX and what isn’t might be due to your mistaking all Greek translations of the Hebrew text as Septuagint, where that is not the case. If it wasn’t on a scroll written by Jews circa 200-150 years before Christ, it is not the Septuagint.