It's confusing because it was stated wrongly.
https://en.wikipedia.org/wiki/Yes_and_no#The_Early_English_f...
Yes contradicts the negative question. So "Is this not a mistake?" should be contradicted with "yes, it is a mistake" or affirmed with "no, it is not a mistake".
It's further confusing because we have the idiom of suggesting things politely in a tentative manner such as isn't this a mistake? which has lost its sense of negativity and has come to mean "this is a mistake, I think," as opposed to being parsed literally to mean "this is not-a-mistake, I think".
I'm trying to parse the extra "not" in archaic holdovers vs plain modern English. It seems to carry a subtext.
Modern "Are you happy now?" is said with sarcastic tone, to spoil happiness. Would be archaically "Are you not happy?" As if to dare contradiction. It's loaded, unlike when saying sympathetically "Are you unhappy?"
Others:
"Are you not entertained?" "Are you not the very same Smith that dwelt at Haversham?" "Prick me, do I not bleed?"
But commonly: "Are you not a Christian?" most likely seems direct, but said in a formal sense, "rhetorically", an exhortation to act like one.