Ok, I figured it out with the help of reading the notes!
I don’t know what they mean by “…until the UI is fixed…” In any case you use iocage commands to set the password in a persistent way. Don’t use the web UI in Duplicati to set the password, it won’t stick between plugin restarts:
By the way, I am seeing that Jails on FreeNAS (and BSD I presume) are extremely similar to Docker containers. They are way more than just the chroot jails I was thinking…