The class javadoc for PluggableMBeanServerBuilder obscures the way JMX integration actually works by discussing passing the -mbeanserverbuildermodule=org.jboss.as.jmx arg to JBoss Modules.

The way this actually works is the main module JBM loads (e.g. org.jboss.as.standalone) imports services from the org.jboss.as.jmx module and that module includes a META-INF/services/javax.management.MBeanServerBuilder file that specifies PluggableMBeanServerBuilder. org.jboss.module.Main uses ServiceLoader to find PluggableMBeanServerBuilder.