[She's never been in a relationship with Jack. Not really. They had their marriage and for that day it felt very much like they had been and it had it's ups and downs but they were largely great together. There was nothing saying that's how it would really play out though. And then timing was never on the side of them being together. He was faithful to Ianto during that window when she might have chosen differently than Chekov. She has no regrets there. She is extremely happy with her Russian space man.
So, when Ianto speaks like this she knows a little of his side and a little of Jack's and then there's just knowing them both and she doubts Jack ever meant to make him feel that way, but not that he felt it.
She leans over and kisses the top of Ianto's head.] Anyone would be lucky to be with you. You know that, right?