That's just vandalism.
what a waste of an old specimen.
i've seen this on old western movies. usually the indians are portayed as resorting to this water source when others are unavailable.
here is some info i found:
http://www.desertmuseumdigitallibrary.org/public/detail.php?id=ASDM08448&sp=Ferocactus%20wislizeni
''NotesThis is the barrel that the Seri and others prefer to use for emergency water; Seris have survived on it for as long as a month. The juice is flat-tasting but not completely harmless. Drinking it on an empty stomach often causes diarrhea, and some Seri experience pain in their bones if they walk a long distance after drinking the juice.''
but i agree, he just doesn't seem to know what he is talking about.
If only he was in Africa and that was a Euphorbia he was decapitating/drinking...