nixbitcoindev explains here that the additional security provided by nix-bitcoin is due to minimalism, code reproducibility, compartmentalization and defense in depth.
nix-bitcoin uses NixOS which is a purely functional operating system which means that it builds the entire operating system from the source code of every application of the Linux kernel resulting in the same system every single time it is deployed.
With a general Linux distribution it is not possible to de-install all the packages you don't need as it would eventually result in a broken system. NixOS has a kind of formula where it goes through and calculates what you need and only builds that. This minimalism significantly reduces the attack surface.
Everything is reproducible with NixOS, not only Bitcoin related software such as Bitcoin Core, c-lightning, lnd etc but the entire stack including the Linux kernel. As a result you know you have exactly the same system that everybody else has. That’s a really strong defense against the developers inserting malicious code or code upstream getting compromised. Once the developers have verified the software, within the hash it’s going to be the same software that everybody is using.
Every service runs in its own little box under its own user. It can only see its own directory and with network namespaces it can't even scope out your entire network. It can only scope out its own network and its Linux namespace and the ones it is allowed to see. Outside processes outside of that network namespace also can't look inside. That offers a great deal of security because now programs like Spark wallet that connect to your c-lightning, they’ll never see JoinMarket, they’ll never see Electrum, they’ll never see bitcoind. They are maximally compartmentalized.
Defense in depth means putting up multiple walls or multiple lines of defense. nix-bitcoin isolates by users and isolates on the network level with systemd.