Should nixos-generate-config detect LVM-on-LUKS?

I’m exploring NixOS for the first time and confused about the installation process. The manual says:

The LUKS volume should be automatically picked up by nixos-generate-config

but I’m not seeing that happen. I’ve tried this in NixOS 21.05:

parted /dev/disk/by-id/ata-VBOX_HARDDISK_VB9fd13ce8-0e7b0319 -- mklabel gpt
parted /dev/disk/by-id/ata-VBOX_HARDDISK_VB9fd13ce8-0e7b0319 -- mkpart pv-enc 0% 100%

cryptsetup luksFormat /dev/disk/by-partlabel/pv-enc
cryptsetup luksOpen /dev/disk/by-partlabel/pv-enc pv

pvcreate /dev/mapper/pv
vgcreate vg /dev/mapper/pv
lvcreate --name root --extents '100%FREE' vg

mkfs.ext4 -L root /dev/vg/root
mount /dev/disk/by-label/root /mnt

nixos-generate-config --root /mnt

The resulting configuration in hardware-configuration.nix is:

imports = [ ];

boot.initrd.availableKernelModules = [ "ata_piix" "ohci_pci" "sd_mod" "sr_mod" ];
boot.initrd.kernelModules = [ "dm-snapshot" ];
boot.kernelModules = [ ];
boot.extraModulePackages = [ ];

fileSystems."/" =
{ device = "/dev/disk/by-uuid/8be4a162-081d-4605-9dd3-f1b8a2008f07";
    fsType = "ext4";

swapDevices = [ ];

virtualisation.virtualbox.guest.enable = true;

I know I could manually add boot.initrd.luks but I’d like to understand why the automatic detection isn’t working as I expect.

Personally I’d find it much more convenient if there was a tool that would generate a script with partitioning commands from a hardware-configuration.nix.