This is all platonic, but she knew that already. Cassian is just reaffirming what she'd already been thinking.

"I won't avoid you forever." Just until she gets over this stupid crush, convinces her heart to stop feeling anything. It's stupid anyway, it's probably just proximity, the fact that she can only be herself with him, postpartum hormones. There's a lot of reasons to convince herself it's not real, that she doesn't like him.

It's meant to be an assurance, in any case. That she isn't going to ignore him for the rest of this, that she'll still parent Rey with him. She just needs space to stop liking him so this weirdness can go away.