TL;DR — Ghost started out as a WordPress alternative but gradually moved to focus on newsletter formats (at which point, see u/maegul’s reply), and the dev team have worked on implementing ActivityPub for a good while now.

From what I can tell Ghost is a really mature project with a lot of traction. I think having a federated newsletter platform is an interesting addition to the fediverse.

A few months ago, the “Nazi” presence on substack and substack’s insistence on not moderating them (like at all it seemed) broke as a story, during which Casey Newton (and by extension his “platformer” blog) got engaged with substack about the issue and, after being disappointed with substack’s responses and policies, famously left for Ghost (see their post on the move here.

Pretty sure that boosted its profile and prompted talks of federating, which they were initially hesitant to do … but here we are now.