It is a joke, as so many interviewers complain that people lie about their technical skills and experience, if you listed String as a technical skill and the interviewer thought you didn't know what a mutable String was then they would conclude that you lied about your skill set. Hopefully you can see the humor. I feel your pain so can understand why you might not think it is funny.
Interviewer: What is the difference between a mutable and immutable string?
Me: A mutable string can be changed. A immutable string cannot be changed.
Interviewer: Nope, think mutations. Mutable string can be mutated.
Me: Mutated? Altering DNA?
Interviewer: If you don't know this there isn't any sense in asking more difficult questions.
Me: Floored.