I never asked anything, this muthafukka came in explaining shyt that he probably read ahead.
I speculated that if this kid was karkstarks son and rob killed karstarks that when the kid finds out he'll let Theon go. And some douche comes in and confidently says it's not Karstarks son
yall taking up for him because he said "you should ask yourself if i'm a liar" or whatever, but minutes before that he told Theon he wouldn't cut his finger if he guessed correctly. All indication was that he guessed correctly and still cut his finger because he was a liar....not that he was lying about who he was.
That's the way I read into it, I just rewatched the part and never does he say he was lying about who he was.