When we see that the LXX translators were working from a Hebrew Vorlage different to the one survived to us via the Masoretic textual route, we notice that the anarthrous practice (=KYRIOS not HO KYRIOS) is as frequently applied as when the two traditions are in harmony. Here's an example of what I mean from Psalms 98:1, where the LXX translator seems most likely faced with 2 x Yahweh to translate:
O sing to the L a new song,
For He has done wonderful things,
His right hand and His holy arm have [fn]gained the victory for Him.
A Psalm. Pertaining to Dauid.
Sing to the Lord a new song,
because the Lord did marvelous things.
His right hand saved for him,
and his holy arm.
Other examples exist, such as what we find with Judith, which Perkins mentions as including the distinction between human and divine rulers (ho kyrios=human ruler; kyrios=God).
We are not just saying that Kyrios predates the Christian era, we are noticing that anarthrous (non-articulated) Kyrios predates the Christian era.