There is actually no other difference than the addition of the previous `POST /register` call. The original flow that you proposed was basically the SMS MT flow. This PR addition was to address Jonas' proposal at https://bugzilla.mozilla.org/show_bug.cgi?id=988469#c5 to allow multiple verification mechanisms and to delegate the decision of which verification mechanism the client can do to the server.
Even if we are going to focus on SMS based verification mechanisms for MVP, we still need to differentiate between SMS MT and SMS MT+MO. And having the API ready for other verification mechanisms (like the network based one that will not require any client change) doesn't hurt.