It is, but it’s not recommended for productive use yet. Quote from their newsletter March 2nd:

What about self-hosting?

Several people have asked about using ActivityPub if you’re hosting Ghost elsewhere. Some quick notes about that:

Importantly, Ghost’s ActivityPub service is already out in the wild, open source, and released under the MIT license. We build in public, and all our work is up on GitHub for anyone to download, fork, run or deploy if they want to.

What’s missing right now, mainly, is documentation. You could already self-host ActivityPub if you really wanted to, but there’s a lot you’d need to figure out to get it running properly.

So the question is less “when will it be possible” and more “when will it be easy”?

At the moment we’re moving quickly and making many breaking changes each week which aren’t backwards compatible (like switching to a new DB) – so the app isn’t stable. Even if we did document everything, if you self-hosted then it would just break constantly — so it doesn’t make much sense for us to try to document and promote self-hosting, because it won’t be a good experience for anyone.

Right now we’re prioritising developing the app and building features, deployed in a single location, so we can make progress. Once the app is stable, then we’ll start documenting (and optimizing) the process of deploying it and hosting it elsewhere.

We’re hoping to get to that work some time this summer, and we’ll share details of that here, as we go. Our first priority is just getting ActivityPub working and stable with a base feature set.