I don't think Noel was Bound to anyone at the time, but during that time, her sister was overprotective and kept her by her side at all times, which made Noel feel left out, and I think Nael noticed, which is why I think she allowed her to get thrown by Arsa.
Later on in chapter 10, we see that Nael has really good senses if she hones in on them, so During the time of the Arsa attack, she could have been honing in on her senses to see where she would have ended up, and while not being able to tell who her partner would've been, she didn't care cause I believe she wanted her sister to find a partner who was the worst kind of human, so her sister would feel as if she needed Nael.
So when they encountered, Nael would have a Justified reason to kill him and take back her sister, she hoped Noel would grow more attached to her. This is just speculation, but it does hold some weight.