This is a bit out of date.  The isinstance() docs now specify that the predicate tests for direct, indirect, and virtual inheritance. Likewise, the super() docs already specify that only the __mro__ is searched.  So those docs are already precise.

Am thinking of adding a FAQ entry about when direct inheritance is required and include the OP's example.  FWIW, this isn't really about super().  It affects any attribute lookup or any other property of inheritance.  A true result from instance() doesn't imply that actual inheritance has occurred.