Dylibs is for including 3rd party code at build time, maybe a client to consume an API.
ExtensionKit allows you to expose realtime UI or information, coming from an installed app instead of a 3rd party service.
That way you don't have to expose your API or build an SDK. You just have functions in your app that can be invoked.
Also the feature is only provided if the user has that app installed. So the experience is curated.