Men are braver than women! The American's reaction is proof of that!
The snake had slithered across my foot before that happened and I did not panic!
Mrs. Wynnes remains calm as she notices the snake slither across her foot. She does not panic like the colonel had thought that a women would do. Her gender does not affect how she reacts.
The readers know that a snake is under the table, but most of the characters don't. Only the American, the servant, and the hostess know about the cobra.
The colonel had thought that the American's reaction proved his point that men have better control than women in a crisis. Also, he thought that women would panic or scream. This was proved wrong by Mrs. Wynnes' calm and brave reaction to realizing there was a cobra.