I see two issues:
1) pluginObjects is not queryable in the standard way. (you would use the 'queryPluginObjects' flag instead)
2)the pluginObjects flag takes two arguments: [string, bool]
so the end result would be something like:
pm.modelEditor('modelPanel5', edit=True, pluginObjects=['', True])
the docs suggest the return value should be the string name of the editor.