As a supernatural big wig, I would have expected Langtry to know who the current KotC are, but he might not have seen a recent photo of Michael. He could also have been testing Harry, seeing what Harry knew of the situation and if this was some plot on his part. Feigning ignorance to make a person tell more information is a common interrogation tactic. Colombo loved it. If Harry didn't know who this Knight was, Langtry might have played things differently.
As for Harry saving the Sword, I recall the argument against Harry wasn't that he saved the Sword, it was his own stupid actions caused it to be in peril to begin with.
I'd also expect him to know which Sword is Am (since it's the only Broadsword among the Three) but he still asks Harry.
I don't think that the Swords and Wizards cross paths very often- and it's also fair to remember that they seldom are in one hand for long.
(Really though, not knowing about the guy that punched out a Freaking Dragon is pretty weird, now that I think about it. Langtry had to be pulling something there, right?)
