An army can argue amongst themselves as many voices or as a unit in a single voice. I was referring to the army arguing against another army, not internally. Your point about the other noun (people) is well-taken.There's a big difference between "swathe of society" and "army of people", which is that in the first phrase both words are singular, while in the second "people" is plural (it's the plural of 'person').
So it's far easier to say An army of people argue than A swathe of society believe . This is in addition to the fact that if they argue, they're obviously divided; if they believe, they would seem to be of a single mind.