This is my question as well. How do they know that these DNA samples are not of the people who prepared and/or maintained the casts?
Ancient DNA has certain characteristic damage that changes the sequence slightly, and you can use that to filter out any DNA that doesn't have that signature. Plus, you can maintain DNA profiles of all people involved in handling to ensure it's not modern contamination from the field workers or lab techs.
Thanks for clarifying.
3 people downvoted me for the honest question.