vietits is correct. To see if I provide some more understanding: 'Ext.require' loads the class, thus instantiating the singletons Ext.Msg and Ext.MessageBox. So, they are immediately available.
If the class has never been loaded (i.e. no Ext.requires), then the singletons are not yet available. So, the load will occurs on the 'Ext.create' call; that is when the singleton would be available.
Hope that helps.