The actual meaning of this saying is that if there is a predefined exception to when a rule applies, eg. you can't post on kc on tuesdays,
it confirms the existance of the rule that that is an exception to, eg. the afformentioned exception implicitly confirms that you can post on kc on all other days
It doesn't mean that if some rule sometimes randomly fails to apply then that confirms & affirms the rule - that would make no sense, because random exceptions have the opposite implication, they make it questionable if there is a rule at all