The unprofessional act that the Man is describing is Serius' leading a "heroic" cavalry charge into a machine gun nest. A few lines down he elaborates as follows:
He did it like an operatic tenor—a regular handsome fellow, with flashing eyes and lovely moustache, shouting a war-cry and charging like Don Quixote at the windmills. We nearly burst with laughter at him ...
Under normal circumstances, such an act would be abysmally stupid. When soldiers riding horses charge machine guns, the machine guns will easily destroy them long before they come within sword range of the soldiers manning the guns.
In this case, the charge succeeds by accident. The opposing gunners had been supplied with the incorrect bullets and are thus incapable of firing their weapons. This bit of blind luck, though, does not change the general strategic truth that a cavalry charge against machine guns may look and sound dramatic and romantic, but is tactically a very silly and unprofessional move.