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 <A HREF="https://hal.inria.fr/hal-02424283/file/44-BeTh81-surveycyclesdigraphs.pdf">Cycles in digraphs - a survey</A> Bermond and Thomassen cite the 1973 paper as follows:

> Meynal, H., Une condition suffisante d'existance d'un circuit hamiltonien dans un graphe orienté, *J. Combinatorial Theory B* 14(1937), 137–147

  [1]: https://i.sstatic.net/2Ho1W.png