So I'm a bit confused by the last part of this post. The protocol is definitely more than a year old, ActivityPub is not new. By any means. And ActivityPub builds on top of protocols that came before it.
https://www.w3.org/TR/activitypub/
https://www.w3.org/TR/activitystreams-core/
http://martin.atkins.me.uk/activity-streams/
As for the posts that appear in the /m/random magazine, they are basically the same as looking at the federated tab on Mastodon - they are posts that arrive here, because someone else on this server follows that person, or they were written by someone on this server. No posts will magically appear on the server without someone here actively following either a person or a community on a remote server. They appear in random, because they are not attached to a magazine, though if they use a hashtag that does track that hashtag, then they will be added to the microblogging tab of that magazine, instead of being shoved in /m/random