ἵνα ἐν τῷ ὀνόματι Ἰησοῦ πᾶν γόνυ κάμψῃ ἐπουρανίων καὶ ἐπιγείων καὶ καταχθονίων,
2:10 so-that in with~the name of~Yeshua, every knee should bow, of~heavenly [ones], and of~on-earth [knees], and of~subterranean [ones], (~robin)
There's no reason why the later use of Τάρταρος (Tartarus), by another NT author, precludes that this is not what Paul was referring to. Granted, it's an assumption that this is what Paul meant, but it's also an assumption that Paul didn't have this in mind, just because the particular word description (Tartarus) isn't used until much later. I also have reservations against the doctrinally imported assumptions about this being sphere of the dead, because one would have to also assumed a great number of other religious concepts about hell, hades, purgatory, and such ...
That is, the original question was, "To whom does the adjective "katachthon" refer to? And the most honestly logical answer would be ...We really don't know!
We do know, however, from the context that they are to be distinguished from those with heavenly (A-GPM) knees, and those of us with on-earth (A-GPM) knees. That is, I think the original question is somewhat misdirected when it asks "to whom" ... because the verse is not really talking about, or making a distinction about "whom," but rather simply saying that "every knee should bow," no matter where that particular knee happens to be.