The motto of the group that leaked it is "A hacktivist group protecting artists' rights and ensuring fair compensation for their work", so my guess is that they're trying to give the press and researchers a look at what goes into the sausage, because the industry abuses the hell out of creative talent.

One question is whether there is one massive slack, or multiple different ones. I'd certainly hope that sensitive stuff is limited to a separate slack, for extra insulation.

They mention a name, and a google search shows that person works in Disney IT, so maybe their credentials were leaked and they had admin access to the slack. In that case, relying on slack permissions to limit the scope of a breach isn't really going to work.