Your interpretation is possible but I think it's unlikely. It's reading a little too much extra into that conversation, in my opinion.

You're saying Taravangian's two sentences are linked in meaning, which is possible but not clear. But then you're saying Szeth's response is sarcastic in a petulant kind of way. I think that's stretching it a bit far. Szeth is hardly ever sarcastic, and it would be out of character for him: he's usually literal to the extreme. Even if that weren't so, reading his response as sarcasm sounds a bit odd, almost like a kid saying "I know you are but what am I?"

I don't think Brandon writes unclear conversation like that. Most likely "Lifebrother" is a character he has outlined in his notes that he plans to expound on later in the story, and this is just an early namedrop to hint at his existence.