The thing is that even true stereotypes can be harmful. If 90% of group A like thing 1 and dislike thing 2, and you form a stereotype on this basis (a true, accurate stereotype) - you offer As 1s when they come round, you market your job that involves a lot of 2s to Bs rather than to As - then you can exclude and harm the 10% of As who like thing 2 and dislike thing 1. It's not being false that makes a stereotype bad.