The definition you references allows that flattery could be false or insincere; therefore, flattery does not have to be false. It could just be insincere. One way it can be true but still be flattery is that it could be given too frequently or in too great a measure. In that way it can reach the point of being insincere. Also, even if it is true, if a compliment is given for the purpose of getting something out of it, it changes from a simple compliment to flattery. That is my understanding of the word. That doesn't directly answer your question; if I had to choose, I would say (2).