Author here. It means that you have to find input that triggers a memory bug to do something malicious. If my dc had the `!` command, then you, as an attacker

Author here. It means that you have to find input that triggers a memory bug to do something malicious. If my dc had the `!` command, then you, as an attacker with low privileges, might be able to modify a dc script to spawn another command that you know can be exploited to get root. Without it, you have to exploit my dc for ROP or some other attack. Of course, then you would just target a shell, right? And that's the point: I've put in enough work, and my bc/dc are locked down enough, that attackers would probably find other ways in.