Why does the key takeaway to this question contradict the explanation for the correct answer?

I've read most of the responses to these posts, and they all say we get to the correct answer by assuming "quite certain" means 100% true. If John's position is presented as a fact, how/why does that mean the position must be true? It can be 100% true that a person believes "x", but that would not force "x" itself to be 100% true. The key takeaway notes for this questions say when someone claims or says they believe something, that means there is doubt.