Correct it's not part of ActivityPub, but it supports a bunch of IndieWeb niceties, like:
The public version version of the instance is really a personal blog/microblog that supports ActivityPub + IndieWeb. It's quite different from regular "fediverse" software.
i. The single-user part is really strict/part of the design, as there are literally no concept of users, everything is assumed to be authored by the instance actor.
I do host multiple instances on the same server (like @firstname.lastname@example.org), but if you're looking to support more than a couple of users, I think it's not the right fit.
ii. And poetry is used because pip alone is not really enough in term of features to lock packages. It tries to implement a "modern package manager" in Python. It is working well so far.
I hope this helps!