Your linked post does seem to be a viable alternative since in fact once you have completed the steps your extra storage will be mounted to /home/user-data/.
The link breaks it, however since that would be the actual location, you’d be fine.
Alternatively, you could do a fresh install restoring your current back up … and setting the STORAGE_ROOT as @joshdata indicated in that thread before the part you actually quoted. This would be the path of least resistance, IMHO. However if you are comfortable with *nix enough to take the steps listed there, knock yourself out! But if you are not proficient with *nix, I would suggest taking my path instead.
Oh, and an additional thought if you chose my suggestion … spin up the new DO server, install MiaB, rsync /home/user-data/, rerun sudo mailinabox, then test … that way you don’t need to mess with restoring backups, and such.