The DNC is the organization behind the Democrat party. Whether it’s a private or public entity as to legal form and status is irrelevant: it’s one of the two historically significant political parties in the US. Its data and internal communications, like those of any political party, are inherently sensitive and could, if misused, precipitate political crises.
Think about the equivalent situation in an Australian context. Much goes on within the confines of a political party that is never intended to be made publically known. The more corrupt the environment, the more potential toxicity that information has.
Now vomit that information into the public domain in an election cycle and see whether it has NatSec implications…