Just to be clear, that's not in conflict with what the author claimed. You can't call that method using a pointer to the derived class. That would be an access violation, because it was declared private. You need to use a pointer of the superclass type.

    struct I_A {
      virtual void foo() {}
    };
    
    struct A : I_A {
    private:
      virtual void foo() {}
    };
    
    int main() {
      A a;
      // a.foo(); // compile error
      I_A& ia = a;
      ia.foo();   // ok
    }

To be honest, I find this behaviour useful so it doesn't upset me. It's a decent way to mark off functions that you needed to implement to satisfy an internal interface as not-for-users.