I guess this is the declaration they are referring to?
The only reference I find to Malvinas in that declaration is point 13:
- Regarding the question of sovereignty over the Islas Malvinas / Falkland Islands, the European Union took note of CELAC’s historical position based on the importance of dialogue and respect for international law in the peaceful solution of disputes.
Which actually uses both names "Islas Malvinas / Falkland Islands" and doesn't really take any stance on it other than "taking note" of the CELAC's position on the conflict.
So it seems The Guardian's only bother with it is about the EU acknowledging the existence of the issue and using the Spanish name of the islands together with the English one?