@countablenewt The problem is that Bluesky Social PBC are the owners of ATProto. From Wikipedia:
Bluesky Social, officially named Bluesky Social PBC, is a privately owned for-profit corporation.
Bluesky unveiled open source code in May 2022 for an early version of its distributed social network protocol, Authenticated Data Experiment (ADX), since renamed the Authenticated Transfer (AT) Protocol.
Note the wording: "...its distributed social network protocol..." ie, ATProto is owned by Bluesky Social PBC.
And, it's easy to see this ownership by just looking at the Github Repository for the ATProto reference implementation.
Compare that to ActivityPub:
ActivityPub is an official W3C recommended standard published by the W3C Social Web Working Group.
And, it has been a standard that has been developed under W3C going all the back to its beginnings around 2016.