He's correct. France steal this piece of SA, so It's a true FRANCE and not embassy, if you born there you get France passport.
An embassy is a piece of territory of X country in Y country. Like Guiana, both are subject to the constitution of X country, and not Y.
As a matter of international law, an embassy is not ''territory'' of the sending state; it is territory of the receiving state that is accorded, through various treaties and customs, some immunities from host- country law.