Do we have enough evidence to actually claim that this has ruined her plan?
She has done a bit of "the plan is A" "A doesn't work out" "ah but the plan was actually B, I just wanted people to think it was A so that B would happen!"
We don't know how much of that was bullshit versus real.
When she goes to talk to Nathanos and move on with the next stage of her plan, it sounds like this was all expected. Weaken the armies with fighting, let them unite over hatred of you, then lead them to their doom while they chase you. That being said it looked like she knew she screwed up when she said it. Unless she was acting?
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He didn't plan for her to say that. He also didn't plan to beat her. He literally says "1 person dies instead of hundreds/thousands/whatever" before doing it.
What he expected was she'd beat the shit out of him and that it would make everyone else reconsider following her when they saw him get massacred.
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It isn't intended to end war-sized conflicts. The two people battling for warchief are both supposed to support war instead of being a coward.
Strength and honor is the moto for a reason. Strength comes before honor. A warchief must be strong.