The comment at the start of DCmd::register_dcmds is out of date since JDK-8373441 removes the "DCmdFactory::_enabled" feature.

src/hotspot/share/services/diagnosticCommand.cpp

void DCmd::register_dcmds(){
  // Registration of the diagnostic commands
  // First argument specifies which interfaces will export the command
  // Second argument specifies if the command is enabled
  // Third argument specifies if the command is hidden
  uint32_t full_export = DCmd_Source_Internal | DCmd_Source_AttachAPI
                         | DCmd_Source_MBean;
  DCmdFactory::register_DCmdFactory(new DCmdFactoryImpl<HelpDCmd>(full_export));
...