The new Account Activity API is currently in beta as the service is tested and expanded. At the moment the only events available are Direct Messages, but more will be added in the future. We do have a sample application on Github that is aimed at using the API with our new Direct Message features.
We are onboarding new developer use cases on a gradual basis, and appreciate your patience.
The limitations you refer to will be lifted in the future when the platform is ready for general availability, and you should expect tiered access to the features.