A black tie wedding is a formal event where everyone is expected to behave in a certain way, whereas a laidback person is someone who is relaxed and not concerned with formality so to say the girl was as laidback as a black tie wedding is to say the girl was uptight.
Consider the statement "The girl was super uptight.". Is the statement "The girl was as laidback as a black tie wedding" false? Yes/no/unknown