It’s a bit clearer in the FAQ
‘** **To uphold the right of Union citizens to move and reside freely within the territory of Member States under objective conditions of freedom and dignity (Directive 2004/38/EC), and to safeguard citizens of the Union from use as bargaining chips in negotiations under Article 50 TEU, requests the Commission submit a proposal to retain the rights of EU citizenship for all those who have already exercised their freedom of movement prior to the departure of a Member State leaving the Union, and for those nationals of a departing State who wish to retain their status as citizens of the Union.
They recognize that EU Citizenship rights are contingent on someone being a citizen of a member state but they want to extend these to all people who were Citizens of a member state even after that member state leaves the EU.