仕様
When a Create activity is posted, the actor of the activity SHOULD be copied onto the object's attributedTo field.
A mismatch between addressing of the Create activity and its object is likely to lead to confusion. As such, a server SHOULD copy any recipients of the Create activity to its object upon initial distribution, and likewise with copying recipients from the object to the wrapping Create activity.
実際に異なる値を受け取った時のことは書いていない。
特にケアしていないように見えるなあ。
PeerTube specifies an array, containing both the author and channel.
attributedToが一つとも限らない
attributedToにactorが含まれている、ぐらいをチェックすればいいのかな