This is part of an overall effort to eliminate non-optional dependencies on the org.jboss.as.remoting module. Both because depending on extension modules has a code smell but specifically because org.jboss.as.remoting module brings in fairly large dependency tree.
In the EJB subsystem there's deployment descriptor parsing that isn't limited to any particular subset of the EJB3 resource tree and that requires AbstractOutboundConnectionService, and hence the org.jboss.as.remoting module.
Better if that code uses an interface from wildfly-network so there is no hard dependency on org.jboss.as.remoting from the root ejb subsystem resource. If the deployment actually specified an outbound connection, then the connection resource would have to be configured for deployment to succeed, but that's already the case.
AbstractOutboundConnectionService doesn't require any dependencies not already needed by wildfly-network so creating the interface doesn't expand the footprint of its module.
Exposing the AbstractOutboundConnectionService impls via a capability is general best practice