> [...] I cannot envision a scenario where that would be possible [...]

I don't think it can be ruled out. If I'm reading the code right, it's preceded by this branch of the initial if/elif chain:

        elif (isinstance(numerator, numbers.Rational) and
            isinstance(denominator, numbers.Rational)):
            numerator, denominator = (
                numerator.numerator * denominator.denominator,
                denominator.numerator * numerator.denominator
                )

I don't think there's any guarantee that if `numerator` is an instance of `numbers.Rational`, then `numerator.numerator` and `numerator.denominator` have exact type `int`.  (Note that an instance of `numbers.Rational` is not necessarily an instance of `fractions.Fraction`.)