It means they didn't reference any existing or decompiled code from the original client. None of it is directly infringing on any copyright, though it may be doing so indirectly since there have been plenty of lawsuits for tools that contain no copyrighted information can but can used to facilitate infringement (e.g. a tool that decompiles a game ROM)
I assume you, the player, have to provide the assets yourself, and the game won't run without them. Since the code does not contain the assets, there is no copyright infringement.
As long as the assets dont contain code, they're kind of fair game. The rule of thumb is you cannot redistribute them, but if the person owns a legal copy you can point to them on their local system. It is not up to you to figure out if they're pointing to a pirated copy or a legitimate copy mind you.
