He could perfectly well have done, and if it weren't for the timeline confusion (the fact that he thinks this is a memory but he hadn't met Spike when this memory actually happened) Jack would totally accept that. As it is, he gets another prickle of unease that something's not right about this, but he just can't put his finger on it.
"So... am I going to have to get the team out to deal with you?" he asks, except that's wrong, too, isn't it? There is no team any more; they're all dead apart from Gwen, and apart from Ianto who somehow is and isn't both at the same time. And anyway, why would Spike be here to cause trouble? Spike's got a soul now, and a blue band. The wariness on his face relaxes into a smirk. "Or did you just pop over for a quick fuck?"