THE WoL WENT THROUGH A REJOINING ON A PERSONAL SCALE
The Exarch explains the mechanics behind the Rejoinings. That the balance of Aether in the Source versus the Shards is tipped until the Source is strained to the breaking point, and at last fractures in the fabric of reality allow the properly aspected Shard to join with the Source to rebalance the scales. That the overwhelming aetheric shift from the Shard being absorbed - Rejoined - is what causes the Calamities, and why each has a given aetheric aspect.
The WoL has been absorbing all the Primordial Light of the Lightwardens, and after Innocence, Ryne says that not only the WoL's body is breaking down. Their soul is fracturing. She uses the Blessing of Light to quell it and hold them together as much as she can, but the damage hasn't been undone.
That takes Ardbert. The WoL's shard, at the last, sees the WoL breaking, and rushes to fill in the cracks. To restore balance. And yes, the Primordial Light is spent - needs to spent - in defeating Hades, but it's Ardbert rushing in to fill in the gaps left in the WoL that mends them.
Y'shtola and Ryne note, after the fight, that WoL is mended. They doesn't understand how it happened, though they're relieved.
The WoL is the microcosmos in the macrocosmos. Mended just as the Source is with every Rejoining. And the devastation wrought on Hades their personal calamity.
The WoL went through a rejoining so the Source didn't have to.
And we walked down the block to my car
And I almost brought him up
But you start to talk about the movies
That your family watches every single Christmas
And I wanna talk about that
And for the first time, what's past is past
epiphany
Only twenty minutes to sleep
But you dream of some epiphany
Just one single glimpse of relief
To make some sense of what you've seen