She didn't seem to be lying. She looked fine too, without signs of fatigue or weariness. This relieved Saber and made her question just how much prana she could access with her Circuits.

She assented, folding a hand to her chest, while she admitted with deep gratitude. "I am and I owe this to you, Illyasviel."

That was why she would repay this favor somehow, to Shirou as well. He had also given her so much without being able to reward him in kind. She could at least make Rin victorious, thus she felt that as a small accomplishment.