It takes her a second to figure out exactly what it is that sounds familiar. The idea of being plucked up by the universe? No. More likely that affectionate little nickname that had slipped out all unthinking. He's always been the only person she can let her guard down around. No surprise that there's an echo of it here.

"Does it?" she muses. If this Bucky didn't have any relationship with the Natasha from his universe, it was conceivable that he'd overheard it somewhere on a mission. She knew better than to get her hopes up that there might be something to unlock in his brain, or that he might have been dropped into this universe too. She knows how hard it is not to be able to trust your own mind.

So when she continues, it's just to share why she's said it at all. "I usually just called him James. Sometimes milii moi when we were alone. It must have been on my mind."