Poe isn't a jealous person by nature. It just doesn't occur to him to be. So he doesn't get it, when her congratulations come out stiff. He doesn't get it until she says I'm not going to be a jealous ex about it.

He almost chokes on his coffee, trying not to laugh.

"She's my roommate, Caroline. We're friends." He's not sure suddenly why he's explaining--there's no real reason for him to do it, but he doesn't want Caroline to harbor any bad feelings toward Rey. She doesn't deserve them, and he'd much rather she and Caroline be friends than not.