Django ORM isn't based on SQLAlchemy's core (there were some proposals concerning this idea https://groups.google.com/d/msg/django-developers/0IuJssTt8t...), so I guess using Alembic or other SQLA migration tool would require some frankensteining to make it work (so it listens to the Django models.py definitions) and bringing in external dependencies, while Andrew's proposed solution is to make it native and more automatic.
Exactly, the value here is in linking it directly to the models and apps paradigm of Django. South already does this, and people love it, so it seems reasonable to continue down that path and make it even better.
