The idea isn't that the old gods corrupted Jaina as soon as Theramore was destroyed, or even caused that destruction themselves.
However, that event (and many others in MoP) had great impact in Jaina's ideals and views. This could have made it easier for the servants of the Void to, slowly but surely, turn her against the world.
Just clearing that up. I don't think those whispers were about Jaina either.