Surprised me too! But I think they said the language was included at the 11th hour by the Belgians. They were obviously preempting what BoJo is suggesting in his column.
Naturally. But just like the statements from Cameron saying he wouldn't resign if he lost this referendum, it's a complete fiction. Article 50 already states that there would be a negotiation between a state that voted to leave and the EU.
2. A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.