It does support server to server, but currently it does not play well with Mastodon due to its limited support of HTTP Signatures algorithms. I didn't get bothered enough by this yet to actually fix it on my side.
And there are a number of clients that work with this specific brand of client to server ActivityPub but I wrote all of them. The one that can be seen on the internet is a link aggregator similar to HN and (old) reddit, you can find a demo instance at https://brutalinks.tech.
