Gotcha. I had a feeling something around how Mastodon doesn't support ActivityPub Groups (yet?) would be where things are going on. Congrats on piefed, by the way. I'll start studying the codebase now as I'm keen to understand how server-to-server communication works more deeply than I do now. Sending Announce(?) and fetching stuff from other servers...

When I look at the ActivityPub Note object (via curl -H 'Accept: application/activity+json https://hachyderm.io/@otl/111887721960075860) I see:

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        {
            "ostatus": "http://ostatus.org#",
            "atomUri": "ostatus:atomUri",
            "inReplyToAtomUri": "ostatus:inReplyToAtomUri",
            "conversation": "ostatus:conversation",
            "sensitive": "as:sensitive",
            "toot": "http://joinmastodon.org/ns#",
            "votersCount": "toot:votersCount"
        }
    ],
    "id": "https://hachyderm.io/users/otl/statuses/111887721960075860",
    "type": "Note",
    "summary": null,
    "inReplyTo": "https://ttrpg.network/comment/4965852",
    "published": "2024-02-07T01:59:08Z",
    "url": "https://hachyderm.io/@otl/111887721960075860",
    "attributedTo": "https://hachyderm.io/users/otl",
    "to": [
        "https://www.w3.org/ns/activitystreams#Public"
    ],
    "cc": [
        "https://hachyderm.io/users/otl/followers",
        "https://ttrpg.network/u/Neato",
        "https://lemmy.world/u/ForgottenFlux"
    ],
    "sensitive": false,
    "atomUri": "https://hachyderm.io/users/otl/statuses/111887721960075860",
    "inReplyToAtomUri": "https://ttrpg.network/comment/4965852",
    "conversation": "tag:hachyderm.io,2024-02-06:objectId=123754186:objectType=Conversation",
    "content": "<p><span class=\"h-card\" translate=\"no\"><a href=\"https://ttrpg.network/u/Neato\" class=\"u-url mention\">@<span>Neato</span></a></span> <span class=\"h-card\" translate=\"no\"><a href=\"https://lemmy.world/u/ForgottenFlux\" class=\"u-url mention\">@<span>ForgottenFlux</span></a></span>  I totally get how you feel. One use-case I think of is machine-generated image alt-text. These are often not added to images. But with image-to-text ML models, visually-impaired people could hear a descriptions of images that before were never annotated.</p>",
    "contentMap": {
        "en": "<p><span class=\"h-card\" translate=\"no\"><a href=\"https://ttrpg.network/u/Neato\" class=\"u-url mention\">@<span>Neato</span></a></span> <span class=\"h-card\" translate=\"no\"><a href=\"https://lemmy.world/u/ForgottenFlux\" class=\"u-url mention\">@<span>ForgottenFlux</span></a></span>  I totally get how you feel. One use-case I think of is machine-generated image alt-text. These are often not added to images. But with image-to-text ML models, visually-impaired people could hear a descriptions of images that before were never annotated.</p>"
    },
    "attachment": [],
    "tag": [
        {
            "type": "Mention",
            "href": "https://ttrpg.network/u/Neato",
            "name": "@Neato@ttrpg.network"
        },
        {
            "type": "Mention",
            "href": "https://lemmy.world/u/ForgottenFlux",
            "name": "@ForgottenFlux@lemmy.world"
        }
    ],
    "replies": {
        "id": "https://hachyderm.io/users/otl/statuses/111887721960075860/replies",
        "type": "Collection",
        "first": {
            "type": "CollectionPage",
            "next": "https://hachyderm.io/users/otl/statuses/111887721960075860/replies?only_other_accounts=true&page=true",
            "partOf": "https://hachyderm.io/users/otl/statuses/111887721960075860/replies",
            "items": []
        }
    }
}

So I'm assuming an Announce was posted to the shared inboxes at lemmy.ml, lemmy.world and ttrpg.network... hmm... I better start reading!