Revelations from Patch 6.5: "Growing Light", Part 1

[Lyth]

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The EE provides a slightly different description of events:

'In the Thirteenth, Igeyorhm's power proved too much for the inexperienced Bringer of Light and their one-sided battles eventually resulted in a Flood of Darkness consuming the entire world and rendering it a nigh-empty void.' - Encyclopedia Eorzea Vol. 1, p.213

If the description still holds, then it sounds like Igeyorhm simply found a level 1 Azem on the Thirteenth and killed them before they had the chance to gain strength, and then went on to trick their future allies. It could be a slight retcon, though, and perhaps Golbez is that very shard. Hard to tell without them actually commenting further.