alt Hacker NewsApparently it was video clips originally posted, but it seems like posting unpublished(?) copy of any type can fall under DMCA, which I guess makes sense.
-> [HBO Lawyer] classifies the infringing content as 'summaries of unpublished, character, setting, and plots of a forthcoming series'
A summary can't be a matter of copyright. It's probably just the vehicle HBO chose to be able to identify the leaker, because claiming copyright infringement these days is like a superpower, and then use different hammer to go after them.