There's actually still quite a few unknowns. For example, how does Eden tie into this scenario? Eden is pretty explicitly stated as being the source of the Flood of Light - but it's ALSO stated that Ardbert's actions caused of the Flood of Light. Which is it? Are these things linked? If so, how?

My suspicion is that the Ascians had ALREADY set Eden to ramp up the Light-aligned aether, but when Ardbert slew them, there was no one left to control Eden. With no one left to control it, Eden kept at it, eventually resulting in the Flood. So, the Flood wasn't exactly CAUSED by Ardbert's actions - it was caused by the Ascians, who'd planned to supercharge the world with Light, but NOT to Flood it.

Basically, it wasn't so much that Ardbert caused the Flood of Light, but that he eliminated the folks capable of stopping it (who, incidentally, were the same folks who started it in the first place).