I think they mean that because of the unique process that 3d printers use to create something, stuff that can be made easily on a 3d printer can’t be replicated through other manufacturing techniques, and vice versa. For example, I designed an earring that is 1 solid object, but made up of 3 separate moving pieces; like links of a chain that have no split in them. This would be an impossible task for any other kind of manufacturing process. It would be like making acar engine all at once, rather than having to make the individual parts and then assemble them afterward. You can have gaps and cavities in a print that you could never have in a cast or injection molded piece. But this method means that you also have to worry about things that you wouldn’t using more traditional manufacturing techniques.