(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the אִישׁ term, by employing a situation-oriented construal as outlined in “Notes on Gender in Translation,” pp. 11–16.)
This usage of אִישׁ is conventional: it profiles the referent as a distinct entity within a larger military force—for Israel’s force has consisted of Judah’s contingent plus that of the other Israelite tribes. Although they had been united in rebellion against David, this instance of the term אִישׁ יְהוּדָה now points to a budding and significant distinction.
On the regular usage of singular אִישׁ as a “collective” term in the context of hostilities, see my comment to Judg 7:23. On its occasional use to denote a contingent of military forces, see my comment at Judg 7:24.
As for rendering into English, the NJPS ‘all the Judites’ misses the collective and situational nuances. On properly rendering the collective usage of אִישׁ into idiomatic English, see my comment at Josh 10:24.