Whether or not it was Ramuh who stopped the Garlean advance or not, I DO remember a mention of Ramuh (not by name) being summoned before the events of 1.0. I wish I could remember if it was in one of the missions or if it was just one of the things a sylph floating in the Waking Sands said. It was something along the lines of 'once before we called the primal one and he tainted the hearts of the ones who called him. These ones will not do so again.' But unless someone can provide the text, my memory should be taken with a grain of salt.