Note Mab doesn't actually say those powers of judgement care about this case. She's making Harry care in a way he understands, even though Mab cares far more about imbalance in the scales.
Although I do think subsequent events somewhat undermine the plot point; after Summer Knight Winter is for long stretches of time short on a Knight, Lady, and second-in-command... and only the weakness at the Lady is somewhat mirrored in Summer. And yet, disaster doesn't strike VERY hard (sure, the White Court snipes a bit at the edges and Nic thinks he can flaunt the accords, but overall it's kind of meh).