The advanced help text at /help/ctools_plugin_example/Argument-Plugins--Starting-at-the-beginning says "And note also that the primary function that we use to provide our argument (ctools_plugin_example_simplecontext_arg_ctools_arguments()) is also built from the machine name." but that function doesn't exist anywhere. It looks like it has been replaced by the $plugin array. If that's the case then the docs need to be amended to reflect the code.