Don't know why, but it seems to work this way
http://www.flexthinker.com/2010/03/flex4-moduleloader-and-runtime-skins/. You have to remember to add ModuleLoader ( FlexModuleLoader ) to display list
BEFORE you start loading module.
This is bad situation because in this case new FabricationModuleLoader and FabricationAirModuleLoader classes ( based on ModuleManager ) won't work properly.