Actually, it's completely legitimate for a player-character to roleplay stealing OOCly public information from a minor NPC.
Interesting interpretation of legitimate.
You do NOT get to speak for all nobles and intellectual or cultural elites of a realm. Aurvandil is not yours on an OOC level. Also, you assume Terran is ignorant of Orvandeaux culture– we're really not. Maybe once upon a time, but not so much anymore. Terran isn't ignorant of Aurvandil's culture, it just wants to kill Aurvandil's culture.
Interesting. So you're not culturally ignorant of Aurvandil? Hmm.
Tell me more about how your character knows who Mendicant's Royal Champion is, or if your character knows what Orvandeaux Chivalry demands of our conduct in war, or how your character knows who the Porte-Auriflamme is and where the Auriflamme is kept, or if your character knows why we've left Tovabur rogue for so long, or why don't you tell me that your character knows why we have the Chevaliers Disith, and the Chevaliers Hausos, and what is the integral difference between the two? Oh does your character know why Mendicant is a High Sovereign and a king simultaneously? Does your character know what the name is of the sword we use to acclaim our Monarchs? Could your character correctly identify the three different philosophies embodied by Sarit, Mendicant and Allomere? For good measure, can your character correctly state why the Evanburg duchy is a Marquessate, the Tower Fatmilak duchy is a Margraviate, and Candiels an ordinary duchy, and what their different functions and governance are? Or could your character say why Bol is in the Candiels duchy despite the Margraviate separating it from Candiels?
If you "really not" ignorant of our culture as you boast, then your character will know the answer to each of these, these are the most basic roleplayed parts of Aurvandil and the most frequent at that. I'm sure your character knows well enough about the culture he wishes to destroy, otherwise you don't exactly have a business claiming to be knowledgeable of Orvandeaux culture.
I'm not sure where you get the rest from– I don't know of anybody who has been roleplaying Aurvandil as mentally disabled or militarily weak.
You do, of course, you never say it outright in your roleplays but you equate us to that by your portrayal of Aurvandil.