so I mentioned the same thing, though one thing to note (which I doubt duck dns helps with either). Is one can't use google's dynamic dns with AAAA records (i.e. ipv6). Personally, I think this is massive oversight on their part.
when I go into the web ui for my domain and go to create a dynamic dns name, it only gives me an option for only an A record.
My "guess" is that your solution is to create a normal AAAA record and ddclient can update that record with the info retrieved from checkipv6.dyndns.org? so not using google's UI to setup a AAAA dyn dns record? and my guess is that username/password scoping that you got for host4 worked the same for the "fake static" AAAA record?
or you could have the ability to create AAAA dyn dns records via the UI and for some reason I'm just blind.
Likely because the person/team that built this got their annual bonus for making something new and moved on to other things before ipv6 became relevant in their eyes.