If my T had said that to me, I'd want to know what a secure attachment looks like. I might also want to know how it would benefit me to have the "secure attachment" that T is looking for. How would I feel or behave differently, if my attachment was "secure."

I'm not sure a "secure attachment" is, like, an accomplishment. I don't think it is something you should feel bad about.

I do, generally, think that things I've worked on in therapy related to interpersonal issues do transfer over to other relationships. Or, said another way, my interpersonal relationships have improved over the course in therapy, partly because I've become more self aware and partly because I've been willing to do things differently with others due to my understandings about myself developed in therapy.