Balaam's advice is being expounded on the basis of Numbers 31,16. It is then generalized by Philo (Vit Mos I) and to a lesser degree by PseudoPhilo. Josephus offers a variant of this. It is evident that in the first part of his Balaam - speech he deals with the consequences of the advice in a reductive manner. The seduction of young Israelites or Judeans by female foreigners will not succeed ultemately but will rebound punitively on those who instigated such a temptation to adopt a foreign religion. That might be a repetitive pattern of Jewish history - there are counterparts in other subject matters which Josephus presents in the fashion of such repetitive patterns. It's only in the second part that Josephus turns to the more traditional way of dealing with Balaam's advice.