If a person thinks that his existence is meaningless, then at least there is still the concept of meaning in his mind. If he thinks that meaning itself is meaningless, he just thinks to himself “nothing” and then based on this meaning denies that this meaning has any other meaning than “nothing”. Defining the concept of meaning as “empty” does not prove that meaning cannot “have”. Maybe he just hasn’t found it yet, maybe he has but can’t confirm it. Of course, it’s possible that he’ll never find it, or even if he does, it’ll never be confirmed…