Arquillian ships with a bunch of test instance "enrichers" for injecting objects to annotated fields. Among them are enrichers for Java EE annotations @Inject, @Resource and @EJB. These enrichers are enabled only if the respective annotation (or some other identifying class) is "present on the classpath". Which, for in-container tests, means visible from the classloader that loaded Arquillian. In Swarm's case, that's the module classloader for module org.jboss.arquillian. This module is defined by Swarm itself.
This module currently exports javax.annotation.api, which contains @Resource, and javax.enterprise.api, which contains @Inject (and the BeanManager). For the @EJB enricher to work, an export of javax.ejb.api is needed.