I think this author uses declaration to let the readers feel that this sentence has irony and humour，Am I right? Because the general usage would be "but after I had heard what Lorna said, […]."
Here is the quote:
"I have a 10 o'clock meeting with Patrick Beeman. To discuss what needs to be done." I suppose I nodded. Freddy wanted me to do my Flesh-Eating Zombie thing, but after Lorna's declaration, it was beyond me. She said something else that I heard but didn't really hear, climbed into the car and drove off.