But how you make the leap that that symbolism proves the story false is beyond me. It's asinine logic, in the extreme.
First of all, did it ever occur to you that the protestor could have used lamb's blood specifically to mock its symbolism?
Secondly, maybe he just used lamb's blood because it was what his butcher had on hand when he went shopping for his protest.
It is actually interesting as one of those urban legend things. Sociologically or psychologically it is interesting that such a story is making the rounds.