[nomadking]

The first is a "joint legally binding instrument" on the withdrawal agreement which the UK could use to start a "formal dispute" against the EU if it tried to keep the UK tied into the backstop indefinitely.
The other is a joint statement about the UK and EU's future relationship which commits to replacing the backstop with an alternative by December 2020.

So how is this any different to before? If it is so cut and dried that the backstop is only temporary, why the need for a possible "formal dispute"? If the result of the "formal dispute" goes against the UK, which it probably will, then the backstop becomes permanent.