ShB addresses the First, not the Source. We're going to the place that the previous Warriors of Darkness tried to 'save'. Their home was overwhelmed with light thanks to its own Warriors of Light essentially beating Darkness entirely on their world, to the point where Light started consuming it.
In short, we're going on a holiday to a different world (which will no doubt lead to issues manifesting on Eorzea in our absence) and becoming Warriors of Darkness to combat their Light problem. You can even hear Arbert's "Still it came to this" dialogue in the new trailer, as well as new dialogue from Arbert as well, so we'll no doubt meet the previous WoD's or learn more about them in some shape or form along the way, likely near the start as it was their actions that inadvertently led to the fate of the First.