I guess they would rather the publicity be made when the issue is being talked about in hindsight, studied in a scholarly manner, and at a point where blame could be passed around. Not right now, when there is still a lot we can do to help. Whatever exaggerations there were, the goal was never a secret: to get the world's attention on this issue, and rightfully so.
There IS genocide happening; but for some people, an even more vivid, albeit grotesque, picture must be painted for them to really realize how horrible the situation is. Whatever exaggerations there were were made as an appeal to those people who would otherwise be indifferent to the issue.