Not legally binding: This is not a treaty change, still less a treaty. It is simply a political decision by the “Heads of State or Government… concerning a new settlement for the United Kingdom within the European Union.”
The very first clause says:
Desiring to settle, in conformity with the Treaties, certain issues raised by the United Kingdom in its letter of 10 November 2015…
In conformity with the Treaties. Those will be the Treaties which continue to bind us, then.