The original comment was simply refuting the claim that "X is true" and "I believe that X" have the same meaning. It was expecting you to take at face value "I believe X, but X is not true". Though it seems like that's an inconsistent sort of thing for someone to assert, it is meant to draw out the distinction between the meanings of those two clauses. (compare to "X is true, but X is not true" - a very different sort of contradiction)