Oh, they're doing it. It's been just long enough since their last fight, the one that ended things, for him to come up with plenty more he wants to say. Long enough to realize other things he resents and feels hurt over. Or maybe it's just annoyance at being trapped in this elevator with him.
"No, you've never hidden how much you enjoy sex," He manages to leave a lot unsaid there, puts just the right scathing emphasis on sex to make it clear that it's the only thing he thinks Jack's ever been good at. "I suppose you haven't bothered trying a relationship with anyone new. You were always rubbish at that part and I know how much you hate doing anything you aren't perfect at.
And frankly seeing as there's two of us and one of you why don't you move? It's not like there's anything sentimental keeping you there." Ianto moved all his thoughts out weeks ago when it all happened, even the suits he's not wearing anymore, just to make a point. He left a few deliberate pieces behind like the stopwatch Jack gave him all those years ago when the Warden's decided it was Christmas, the one with his love engraved on the case. He doubts it feels much like 'their' apartment anymore, even with all the years they spent there.