Elsa sighed, Isaac's insistency that she need'nt apologize telling her definitively that she absolutely did. Her confidence in Anna's choices in regards to romantic partners was, admittedly, lacking - that she'd picked Hans not once, but twice spoke for itself as far as she was concerned - but her reluctance on that front should not have stopped her from even giving Isaac a chance before making known her disapproval. He deserved a chance to prove himself, and not to fall in the shadow of Hans and Kristoff.
"I do, actually," she said softly, polite smile in place. "I judged you before I ever met you, and then refused to see what I know now is the truth: That you're a good man, with good intentions. So, for that, I'm genuinely sorry." With a sigh, she stood up straighter, then, the apologetic, meek stance fading into something more athoratative. "That's not to say that, should I feel I need to, I won't step in, but I think we're both anticipating that that won't be necessary."
She didn't mean to intimidate him, not really - she just wanted to be completely clear. No matter how much better she felt about Anna and his relationship, she'd always be extremely difficult to win over when it came to her sister. She hoped that he understood that.