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
Probably it's a whitelist (likely to prevent trolls and such from using fly-by-night instances to spam them). That list can be viewed at https://hilariouschaos.com/instances - then click on "Allowed Instances"
or if they've not allowed piefed where do I see why not?
Unless they posted publicly in a community somewhere, you may not be able to see that - you'd have to ask them.
That said, it's likely the former - just they didn't notice piefed.social until now because it was so small compared to some of the other instances on that list.