It's a grim relatability: resistance until the end. He always knew he was going to die for the Rebellion, so really, the only strange part is he gets to be aware of that after the fact.

He gets it as soon as she says it and understands that she is dead too. The part of him that sounds like Jyn's influence is tempted to say something stupid like welcome to the club, but he resists. "My entire life was rebellion, in some form or another." He does wonder what it might have been like, after the war, had he lived. "You should use this as a second chance. That's what I am trying to do."