You didn't go to fight Garuda with the intent of gaining her power. It didn't even cross your mind, as Alphinaud forced Marques to face the past his mind ran from.
You had a choice, in that moment. Either you pick up your tome and fight, or you don't.
The art of summoning was devised to kill primals. Its practitioners take shards of their broken opponents and convert them into egi with which to battle, stalwart allies in a world where most would succumb to the god-monster's warping touch; those stains of aether that would normally break minds are repurposed for a greater calling.
Situations like this were what your forebears trained for, their wisdom forever lost to the past. It was what you trained for, before the Garleans came and broke the faith you once held.
Alphinaud Leveilleur was a genius and a politician, but he held no true power over you. If you chose to stand down, to step back from this war, he couldn't force you to act. The Scions were broken, the Antecedent gone: the Lady of the Vortex wasn't your battle unless you decided to make it so. There would be other heroes, stronger than you ever were.
But in that moment, there was you, flames stoked by rage burning in your gut and soil sown with love packed around your heart. You made your choice.
(Later, you would wonder if it was right, to have a chance to walk away from this life of suffering and spurn it. Later, Hahavit would tell you the words of the ghost built from a sword and a soul crystal and a broken, furious warrior, and you would feel the weight of the world upon you.)
(But through the lies and betrayal, the pain and despair, you would remember the feeling of that sand-swept church, long before anyone named you a Warrior of Light. The conviction in your heart, Azeyma's sun shining warm through the glass, and your own voice saying I will.)