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.
I could see an argument that it is a trade secret, too-- which could be used to oppose downstream dissemination.
I think a key thing to remember when assessing your own liability is fair use is a defense, not an automatic guaranteed right for blanket uses.
Leaking spoilers of unpublished works can definitely cause market harm, and serves no wider good for the market the same way educational material would.
I wouldn't like to be on the receiving side of this lawsuit. At the very least it's going to be expensive to defend against.