The saying isn't implying that there aren't any things you know you don't know, it's saying that some of the things you don't know are not even conceivable to y

The saying isn't implying that there aren't any things you know you don't know, it's saying that some of the things you don't know are not even conceivable to you without knowing more. You know the question "how long will I live?" And you know this question has an answer which is unknown to you. But there also exist other questions that you don't even know to ask.