This was something which @aaronpk pushed for, and ultimately I think was right. The general idea is that your user can live off-site and have the same inbox/outbox as your posts but *aggregate* the posts from your webpage. "id" is the uri of the json-ld object that is distributed for activitypub purposes, "url" is the uri that existed on the source blogpost.
Of course, I'm not sure if any implementations of AP actually are implemented to support this; they could tho