Excellent question. You can see that the definition of "sincerity" is:
The absence of pretence, deceit, or hypocrisy.
A sincere person believes they are telling the truth, but may themselves be mistaken, deluded, or deceived (by someone else).
In the context of the movie trailer, "Art" is meant to be about things that are true, from any point of view. No matter how sincere they are, a "real" artist must continually strive to break through preconceptions, misunderstandings, and delusions, and produce art that exposes underlying "truths".
Of course, this is still a vaguely sensational statement, meant to attract people to the movie. Whether it's a valid statement, or exactly how an artist produces truth, is for you to decide.