The term "dry drunk" (which I don't like) refers to a person who changes nothing about themselves other than quitting drinking alcohol. They still exhibit all the same behaviors as when they were drinking, but they just don't drink alcohol. Many people believe that in order to be truly sober, or in recovery, the alcoholic needs to address the issues that caused them to drink and seek to change bad behaviors.
The fruitcake analogy simply means that if a person is normally rude, abusive, or neglectful, etc., they will still be rude and abusive and neglectful when they stop drinking. Some friends/family of alcoholics think that if the person just stops drinking, the other bad behaviors will go away, too. Rarely is that true.