Yes. Every new implementation will require some additional integrations in the core server. Atlas (for instance) required a ton of new geographic services.
But I DIDN'T have to rebuild the user ActivityPub integration, user authentication, image processing, ... you get the idea.
By necessity, all Fediverse apps have tons in common. Much than the surface issues that separates them. So #Emissary is that common core, with a programmable layer on top to support different needs.