I would argue that this is not "a Mastodon instance", since it is not running Mastodon - other than that, very very neat work! I'm excited for that "Source Code" link to be live :)
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'
Mastodon is the name of a piece of software as well as an API, a website, etc.
Naming this stuff is hard but calling it a Mastodon instance would be more confusing.
Except "Mastodon instance" means an instance of Mastodon, which is open source. Whether or not it was intended to be deceptive (I'd think a group of smart people would know better), this personally left a bad taste in my mouth.
They federated this brand new code in 9 months, and bluesky still hasn't released anything regarding federation. Don't keep your hopes up, it would kill their business model to let anyone run part of the network. People-driven networks are just not compatible with commercially driven ones, name one successful example.
So is Twitter not a Twitter instance? Like if it looks, walks and toots like a Mastodon, is it not a Mastodon instance?
[1] https://www.wired.com/2013/09/the-second-coming-of-java/
And either way, I think the source code to their Mastodonlike will not be usable since it will be running on their Rama server framework.
So I guess, if you say "it's a Mastodon-clone", you cannot be accused of taking proprietary ideas from Twitter (this is just a guess, they know better).
But technically very interesting and refreshing to see. I really like their approach. It feels they are innovative.