Apparently it was video clips originally posted, but it seems like posting unpublished(?)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 [HBO Lawyer] classifies the infringing content as 'summaries of unpublished, character, setting, and plots of a forthcoming series'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.
This is a legally tricky minefield. Depending on what is in summary, it -may- be subject to copyright but be protected as fair use... but a work being unpublished weighs against (but does not preclude) free use.
I could see an argument that it is a trade secret, too-- which could be used to oppose downstream dissemination.
That too makes sense.