I think that's it. I tried from my own personal single-user pyfedi instance (which has only been functionally live for a few days) and was able to load the community itself onto my instance without issue, but got the same 400 on attempting to actually join/subscribe to it as a user.
Edit: So I think they're just maintaining a white-list or something of the more well known instances, so you may need to get ahold of the admins there to ask them to add piefed.social to the list