NixOS has rm. But it’s not under /bin. There is no /bin in NixOS. The user’s programs are inside /run/current-system/sw/bin, and those are symlinks from files hidden in /nix/store.

Running rm -rf / --no-preserve-root under NixOS, will not nuke the entire system, since /nix/store is mounted as read-only, and only remounted as rw when new applications are downloaded, or older unused ones are wiped.