Fediverse

For clarity, it's not Lemmy that uses 'Article'. I can't remember what does, friendica maybe?
Lemmy uses 'Page' for posts, and 'Note' for comments.
Mastodon uses 'Note' for both, with 'inReplyTo' used to distinguish whether Lemmy would call it a 'Page'. It uses 'Question' for polls.
Pixelfed also uses 'Note', with an 'Image' type attachment (I thinks Loops is similar, just with a 'Video' type attachment).
PeerTube uses 'Video'.
Funkwhale uses 'Album' and 'Playlist'
CastoPod uses 'PodcastEpisode'

There's no one universal ActivityPub server because the Fediverse is based on a broken promise: i.e. that you should be able to use whatever service, to interact with whatever other one. You very often can't, because ActivityPub hasn't been implemented by each platform as some universal thing, it's been co-opted by each to serve it's own purposes. Lemmy best federates with other Lemmy instances, following Lemmy's way of doing stuff, but a good chuck of the Fediverse follows a different model, and is receiving their activity and quietly discarding it because it doesn't know what to do with it. If all the Lemmy instances suddenly chose to use a different protocol than ActivityPub, most people wouldn't notice the difference.