In the linked post they mention "ZFS Native Encryption", but (to save others the time I spent on it): Encrypted ZFS is not available in the installer. However, it will boot encrypted ZFS fine.

If you boot the live image and modify a source file, you can install to an encrypted ZFS. I've documented what I had to do here: https://linsomniac.gitlab.io/post/2020-04-09-ubuntu-2004-enc...