Boromir actually does (vaguely) recognize the title, although it's not one that's been used in Gondor for a long time since only their ruling monarch can perform a knighthood. And what's more, he recognizes her surname.

"I've heard the name Hellsing spoken many times today." It's said in an approving tone, because while he knows that they've been fighting in areas of the city that are badly under attack, he's still not entirely sure what it is or who they are.

He gestures for her to lead, since she knows the way, briefly scanning around for any more threats (not you, monkeys, piss off already).