As far as Dimitri knows, this is the first instance of a timeline where Byleth wasn't a professor among them; however, that doesn't mean it will be the last. According to The Peacekeeper, these things can sometimes occur in waves as they upload from a certain world or timeline. Had his just ended where theirs was already gone? It's much to consider, much more than he can put his mind to right now.

"It's not that," he's quick to interject. "Byleth and I - the male one, anyway - are... partners. And even if you didn't know him well, I'm certain that he'd want to get to know you." More selfishly, Dimitri also wants to keep this version of himself close. He wants to find out as much as he can while the opportunity presents itself.