The Google policy on PII linked from YouTube's response quoted in the article is intentionally a bit broad and vague and considers "full name" PII, but lots of people say their full name on YouTube videos. Showing ID cards isn't mentioned in the policy - those could violate the rules about addresses, but the ID documentation being shown certainly isn't accurate anymore: the people in the videos escaped the camps and I presume aren't in China anymore.
Maybe it's the documenting of the relatives still in the camps YouTube objects to. If this is a violation, would a news clip about a missing child that includes the child's name and photo also be a violation? Seems like it should be. If YouTube takes that down, are they trying to help the missing child?