Illaoi's Test of Spirit and True Damage Interaction

Illaoi's Test of Spirit, where she pulls the spirit from her target, transfers a portion of the damage dealt back to its target. There's something I'm wondering about. I took Illaoi bottom as a support and I went with the relic shield build path. I noticed that I would proc relic shield when the spirit was low and thought it might be a bug. But, I noticed one time that the target got chunked for nearly 400 damage (about as much as my gold item should do as true damage) when the spirit died. I get that true damage can't be reduced, but the true damage isn't being dealt to the champ -- just to the spirit, right? Does anyone know if this interaction with the Targon path of items, or even the true damage, was intended?