Yeah, no. Xichun was already fighting the Night Drifter for an extended period of time before he ended up getting poisoned so she would still scale to his full strength.
Okay, and? No problem with someone who's Level 67 and has a bazillion passives fighting someone who's Level 70.
Your right, it actually starts at level 60(i.e. the highest level we've seen a WAW Abnormalities can go) which Kira is 5 levels above.
Lobotomy Corp Abnos are not comparable to Limbus Company Abnos thanks to unknown Qliphoth Deterrence levels.
In Limbus, Q Deterrence is originally maxed out with the collapse of the branch facilities, which is why WAWs like Ebony Queen's Apple is originally a joke even to Canto I Sinners. As they collect Boughs, the Q Deterrence lowers, best shown in the rematch against Ebony Queen's Apple in Canto IX, where it's now Level 58.
In Lobotomy Corp, Q Deterrence is lowered to some unspecified amount for the sake of letting Abnos kill people to produce energy. We don't know by how much, nor how it compare to the Q Deterrence level of Limbus Company.
Furthermore, Abno Risk Levels as strength scaling is dubious anywho due to how [Random Bullshit Go!] Abnos are in terms of abilities, and their Risk Level is primarily an indicator of energy production levels, as stated by Hohenheim (and is backed up by the game mechanics of Lobotomy Corp. Regardless of their abilities, all Abnos of the same Risk Level produce the same amount of energy, with the sole exception of Pink Heart, which was a Donor Abno meaning it wasn't designed by Project Moon)
Finally, none of this even slightly supports Kira scaling to characters who can one-shot the Sinners. Dante canonically uses Identities in fights, and it's unknown if they spend tickets keeping base Sinners at the level cap. Kira stopping an attack that splatted base ID Don means nothing. And Sancho's stats do not scale to Don thanks to Rocinante, which is best shown in how she's not automatically Level 85 when you start the game.