Damage to property isn't 'violence'. I think the main problem with these kids is that they're elitist. They don't participate in democratic processes, they don't make proposals at the general assembly, and they impose their tactics on the march. And what they're doing is tactically stupid. And they look like idiots in their black hoodies, with their little black flags and their little cans of spray paint.
Look, this chap is basically sensible. He wants to stop a bunch of provocateurs from wrecking the march, and he's right to say that 'violence' is not the same thing as challenging the system. But it would be a shame to foreclose what is an ongoing tactical argument, which must necessarily reflect changing circumstances. In any calculable scenario where the system was in fact being changed, the state would use outright violence to defend it. If police assaulted a mass picket, or a demonstration, people would have every right to defend themselves if they could. Invoking 'non-violence' in such circumstances would be absurd. One should not to turn a valid, but provisional, tactical calculation into a spurious categorical imperative.