Hunters played a role by warning the City of the Fallen attack. This was all-out war, and based on lore, it was a massive battle. I highly doubt an [b]entire[/b] class of Guardians pointedly ignored the fighting as their home was on the verge of infiltration. Some lone Hunters probably didn't partake in the battle, but I choose to believe at least some were ordered to assist by the Vanguard.