I'm not sure why this is funny. Did anyone think Trudeau meant he would engage him in physical battle? What is it about Harper that makes him seem more likely to take Putin in a fist fight? The lego hair? His dad paunch? Ironically, Trudeau is a boxer, and actually worked as a bouncer, so if I had to bet, I'd bet on him over Harper or Mulcair.
What Trudeau clearly meant was that under the Liberals, the Government of Canada would stand up to Russia (to the extent that Russia cares what Canada thinks, I presume). There is nothing in Trudeau's political career which suggests that he backs down from fights (of the non-physical type) - in fact, quite the opposite (he picked a riding that was hard to win, he has doggedly and determinedly withstood years of attacks, etc.) Is there something in the Liberal foreign policy platform which suggests that they would cave into Russia?