Yeah, I think this is just an ActivityPub server that supports the Mastodon extensions, right? I think we should embrace the fact that the federated world can be diverse, rather than just call everything "Mastodon"
And it's a very slim ActivityPub inplementation. For example, I don't think you can do basic things like get an individual post in ActivityPub. This should be easy simple json-ld to get but it's just 404. https://www.w3.org/TR/activitypub/#retrieving-objects
curl -L -H 'accept: application/activity+json' 'https://mastodon.social/users/Gargron/statuses/18614983'