I believe the idea is that the stated purpose of the letter was to recommend someone for a job and the letter itself only describes extranious traits. The law requires that letters of these types only contain favorable information, that is a person used as a reference cannot trash the referenced person to a potential employer thus destroying their chance at a hire. People can sue over things like that--at least that is my understanding. So this article is about implication. You ask "Is this person suitable for a teaching position?" and I answer "He has nice teeth and washes his clothes most of the time." Here it could be taken that I am using true examples to express an implicit meaning, which is "This person is not a good candidate" which may or may not be true.