'Avant-garde', as a military term, is obsolete in English; the English word is now 'vanguard'.
'Skirmish line', to my knowledge, has no accepted non-military uses, but even if it did the interviewee is clearly referring to the military formation and therefore considering the protesters to be hostile enemies.
There's a clear difference between protestors and enemies no matter the amount of hostility. Thinking of the protestors as enemies is bound to escalate conflict instead of deescalating it.
'Skirmish line', to my knowledge, has no accepted non-military uses, but even if it did the interviewee is clearly referring to the military formation and therefore considering the protesters to be hostile enemies.