I have no idea where you got that idea. Nix has built in support for building normal non-privileged Docker containers.
The paths are hard-coded but just like hard-coded /usr/whatever paths in other unix software, that's what the chroot in the container is for.
They (other package managers) have to rely on at least a hardcoded elf interpreter or they don't provide binaries and probably don't even handle external dependencies.
