That's not true though, ShB didn't change the actual properties of Astral and Umbral at all, Astral is still Light and Umbral still Darkness, all that changed was their effects. Eorzeans and those on the Source believed Astral/Light was the 'active' element, giving life, energy and vitality, with Umbral the 'inactive' element, representing death, destruction and decay.

However ShB revealed the Source had the Astral/Umbral elemental aspects mixed up, and that the correct order was actually the reverse (and this was confirmed in EW); Umbral/Darkness is actually the 'active' element, representing action, locomotion and energy, whereas Astral/Light is 'stasis', stability, order and inactivity. But they are still Light and Darkness as they've always been.

So the dialogue and debuff explanations are actually correct and working as intended lore wise.