That means they have access to direct messages of those eight high-profile accounts. Also, they now have their address books, if they synced it.
They said it was purely for Bitcoin and account takeovers and they let it get out of hand. Not sure why we're taking their word for it.
These guys didn't rush to read these DMs in the precious moments they infiltrated the most influential accounts on Twitter.
They now have them in perpetuity, searchable and sellable, as a zip file.
It could conceivably be any large account that didn't tweet during the hack, since they would no longer have access.