You can be quite sure that M. Meyniel means "Monsieur Meyniel" (a common usage in French).
Here is what I think is definite proof that M. Meyniel is H. Meyniel: The acknowledgement of the 1973 paper by M. Meyniel thanks J.C. Bermond, so evidently Bermond knew the author. In the article Cycles in digraphs - a survey Bermond and Thomassen cite the 1973 paper as follows:
Meyniel, H., Une condition suffisante d'existance d'un circuit hamiltonien dans un graphe orienté, J. Combinatorial Theory B 14(1937), 137–147