That's the thing with Social Justice Warriors, they're all fucking stupid, and they truly believe that any deplorable act that they perform is perfectly okay when they do it.
I doubt it's a stunt, because Lauren Southern has gone to these kinds of events before and has been on the receiving end of some horrible comments. She went to a Slut Walk, and when she commented "There is no rape culture in the western world, rape is illegal, and rapists are sent to jail", their response was "Well, I hope you get raped".
But SJWs have done all kinds of shit, even advocating for
philia, so throwing piss on a dissenter is par for the course with them.