She kinda just did that cause she felt like it. From the LN:
"By the way," Carillon (**For some reason it's Carillon in the official translation) asked as he strode up to Milim, "if I could ask you something?"
She smiled back with the Dragon Knuckle eased into her fingers.
"Mm? Sure! Anything!"
"I just wanted to be sure... You weren't under anyone's control? So that was all you when you were whipping the life out of me?"
Carillon was smiling, too, but I could see the veins on his forehead bulging. Yeah, I'd be wondering about that, too.
"Huh?! That, um..."
"It's fine, it's fine. It just means I'm weaker than you. But," he added, no longer hiding his anger, "You willfully blew up my entire nation, didn't you?"
Milim was caught off guard for a momentーbefore immediately raging back at him.
"Oh, come on, Carillon! That's the kind of small stuff you're preoccupied with? What's it matter?!"
Yup, that's the real Milim, all right.
"It's not small stuff! You know I could have died back there?!"
"Oh, don't give me that. Just shut up! I was so passionate about my performanceー Um, I mean, passionate about deceiving Clayman that I was trying really, really hard! It's all Clayman's fault!!"