What authority does Turkey have to declare void an agreement to which it was not a party? None, of course. Erdogan is asserting authority as if he were the Ottoman caliph.
“Erdogan: Egyptian-Greek agreement is null and void,” Libya Observer, August 8, 2020:
…Erdogan stressed, in a speech on Friday, that the agreement signed by Egypt and Greece is of no value, considering it null and void.
He added that Greece has zero right to sign such an agreement, considering that it does not have any maritime borders with either Egypt or Libya.