The art is irrelevant. The problem is that the book directly portrays Frank as lesbian. Which is wrong if and only if the sexuality of the individual remains unknown. It is flatly necessary when coving her diaries when the statement is lifted directly from the diary itself.
The difference being, of course, one is an instance of rewriting history, which is not the case here, and the other is covering actual history which is exactly what the book was attempting to do.
It just proves the case the left makes that the right is bigoted when they protest that their kids may be exposed to the fact that gay or lesbian people actually existed in history.