Lightforged are infused with light.
Demon hunters are infused with fel.
A warlock is a user of fel, not inherently infused with it.
Also the opposite of the Light is Shadow(void)… but a Lightforged draenei being a warlock is more strange in the fact that their people have been at war with the fel for hundreds of years.
Before the Storm is peak saccharine Golden-penned tripe. But I read it. Reading Warcraft work by Golden is almost purely an academic exercise to keep up with the lore.
However - it is explained towards the end. Calia wants to come back that way. She gets an extra special choice for an extra special character.
Along with other the other reasons stated, I believe there is a small mention of her suffering from terrible dreams as a living human, and then being Undead meant she no longer needed to fear dreaming.
According to ‘Before the Storm’ it’s written as follows:
A shiver ran down Anduin’s spine, and he swallowed. He took a deep breath and stepped toward the archbishop. “I’m here, Your Grace,” he said quietly. “What would you have me do?”
Faol turned his face up to Anduin’s. “I’m not quite sure,” he said. “But Saa’ra was insistent that you were to be part of this.”
And then Saa’ra, who had been silent, spoke in their minds.
Calia would come to me when the dreams of what was past were too painful to endure, Saa’ra said. I cautioned her to have patience. There were things she had to do before the dreams would cease, things she must understand. People who would need her help.
And I assured her of this seemingly strange truth: that sometimes the most beautiful and important gifts come wrapped in pain and blood.
The truth of those words hit Anduin’s heart. Those were the gifts that no one ever wanted, that one would do anything not to have bestowed. But they were also indeed as Saa’ra said: beautiful and important.
There will be no more of those battles for her now. Calia Menethil will be freed from the pains of the living, from the nightmares that once rent her heart.
She understood that those on that field were her people. And she accepted that responsibility by giving her life to try to save them. Not human, as they were when she was young, but Forsaken, as they were in that moment.
Light and dark. Forsaken priest and human priest. Together you shall bring her back as the Light and she herself would have her be.
So basically it was the will of the Light, and herself, to have her take the form she got.
The power of the light that courses through a Lightforged Draenei doesn’t protect them once they’re dead. Once a Lightforged Draenei is dead they’re just like any other corpse, and can be raised via necromancy.
Think of it like a cup. A Lightforged is a cup full of light magic. When they die the cup is knocked over and most of the light pours out. When they are raised from the dead by Bolvar as a DK, he basically sits the cup back up and fills it with death magic. A Small residue of Light may remain from their time as a Lightforged, but it isn’t really what they are anymore.
I don’t know if the Light even pours out as Lightforged Death Knights retain all their Racials including a powerful AoE that’s practically a Flame Strike with Light instead of Flame.
They still explode in a burst of Light when killed too. The Racial is called Light’s Reckoning signifying that the Light is holding the attacker to count for killing the Lightforged not that the Lightforged is no longer able to contain the Light within them.
That said one would think that the Light would painfully remove & replace the Nathrezim’s Death Energies(I.E. the Red Death Energies) from the equation in the instant they are raised leaving only Mawsworn(Rainbow-Black), Necrotic(Teal-Green), Mueh’zala(Purple) and Domination(Azure) Death Magic within the Death Knight while keeping the Soul more attached than most Death Knight Souls.
Incidentally I do believe the Nathrezim acquired Mueh’zala’s Purple Death Energies to join the Fel(which they ingested to infiltrate the Twisting Nether) in helping dampen the blow of the Naaru’s Light when fetching Remornia.
That does make sense, though all this makes Lightforging increasingly pointless in the story.
So Illidan would’ve just had his fel powers replaced, like a class change from Warlock to Paladin (as I’ve said to others many times).