Latias is going to be switching in on the Calm Mind. You're assuming it's going to sit there twiddling its thumbs (?) while Keldeo hits it with a +1 rain-boosted Hydro Pump and a +1 super effective Hidden Power, as opposed to the realistic situation of Latias either nuking Keldeo with Draco Meteor / Psyshock or setting Calm Mind up itself. Keldeo is not going to win a CM war against Latias.
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