So these folks bought a book and have figured out how to cut out a page and glue in another one. Seems totally OK.
Now they're selling a kit that helps you "mod" your book by teaching you how to cut out a page and replace it with another one. Still no problems IMO.
Could someone prevent you from running a service where someone mails you a book, you cut out some pages and replace them, and then mail it back? I don't think so.
These aren't great analogies obviously. But it's hard to understand how they have standing given that they sold the copies of the game already. Seems like they'd have better luck going on some kind of contractual violation like their EULA or their agreement you have to subject yourself to in order to connect to the game servers.