Random thought – maybe the fastest way to get full ActivityPub support for WordPress is to offer a feature in any WP site to cross-post to a Mastodon gateway operated by Automattic. It’s possible I don’t understand the problem of course. But Mastodon is open source, so it’s not like depending on another vendor to do this for you. And then you avoid all the hassles with having to say what part of the protocol is ActivityPub and what part is Mastodon’s API.