I think you're mixing "Omniscient" with "Omnipotent" Vallen; the former means all-knowing, the latter is all-powerful. While Tsurugi may not be the former (yeah, long term plans doesn't seem to be her thing), she's very likely still close to the latter despite Kagami saying how she's just a powerless shell. She's been shown to be PERFECTLY capable of taking back her powers, but it's just that she doesn't want to.
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