I think he refers to that you can customize the table Django sets up to handle the m2m which normally is "hidden" (i.e. you don't define it explicitly). But if you want to track for example the time a relation was set up, you can define a custom m2m object that contains a DateTime.

Django Admin is not really designed to be an all-encompassing admin solution I've found, it's a good start but normally you design your own admin console anyway using your API so I've not cared that much about those shortcomings.