There's nothing in his response that makes Terezi certain one way or another of his understanding. He mimics her sign back at her, but she can tell he's only guessing at its meaning. But that's okay. She didn't really expect him to understand in the first place. It was a long shot that she needed to try.

She keeps at it anyway, though, signing as she speaks out loud like Sam would do when teaching her. "Sorry, whatever you are selling, I am not buying. There is no freedom in this place."

He should recognize the sign for what it is now, and her question for what it was. Her answer is plain enough to not need further explanation, she thinks.