According to Seder Olam, the war was over 350 years ago! Surely Binyamin was no longer a small Shevet, for less than 100 years after this, Asa had 280,000 soldiers from Binyamin (Divrei ha'Yamim 2 14:7)! Perhaps this includes Yisraelim living in Binyamin. Many from Malchus Yisrael came to Yehudah and Binyamin after they saw Asa's success (ibid. 2:15:9) - but this is written after the 'census' of Binyamin. (PF)