What do they mean when say “the French equivalent of a Ph.D.”?
According to a 1985 biography in PC Magazine:


  During his college days, Kahn played saxophone in Paris jazz clubs while earning an Agrégation de mathématique, equivalent to a Ph.D. in math.


Another 1985 PC Magazine biography states:


  Kahn, who received the French equivalent of a Ph.D. in mathematics from the French Academy of Sciences, taught mathematics at the university level in Nice and Grenoble before coming to the United States.


Both Wikipedia and Kahn's LinkedIn page state only that he received a master's degree in mathematics from Université Nice Sophia Antipolis in 1975.  This is not the equivalent of a Ph.D., and Sophia Antipolis is nowhere near Paris.

The agrégation de mathématiques is a competitive civil service exam used to recruit high school and university mathematics teachers.  While the exam is extremely rigorous, and agrégés are qualified to teach at universities, the agrégation is not a research degree.  French doctorates underwent a major reform in 1974 and again in 1984, but I believe the closest French equivalent of a Ph.D that Kahn could have earned in the late 1970s was the Doctorat du 3ème cycle.