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.
And this right here is why the “Ghost Gun” thing is largely just a scare like the Halloween drug thing that happens every year. Because it’s generally cheaper and easier to go one state over to the state with lax gun laws and buy a gun there for the price of an Ender 3 instead of going through all the effort of buying a printer and learning how to use it. Same reason why the Mexican cartels smuggle guns out of the US and into Mexico, and not the other way around. Guns are cheap and plentiful in the US, and they’re not hard to get.
There’s some “teh gubernment is cummin fer muh gunz!1!” chuds out there 3d printing guns, but there’s plenty of those people with guns they bought legally as well. The biggest large scale ghost gun manufacturing I’ve heard about is the Burmese resistance fighters who have been printing en masse a design to fight back against a genocidal military coup in Myanmar because the international community has largely ignored what’s going on there and they can’t get guns any other way, which is exactly the sort of situation that design was created for.