I'm just reading the only major recent scholarship on it right now ("Livy's Alexander Digression: (9.17-19): Counterfactuals and Apologetics" by Ruth Morello) and apparently he was working within a broader historiographical tradition that speculated on alternative histories particularly against Alexander. Livy was possibly doing something that would have been expected from his audience (certainly he appears to think so, stating that they would derive pleasure from it - pleasure being something that is secondary to the moralistic tone of his work). Further, he was always prone to patriotic asides. That means that he could possibly have been acting as a corrective to an earlier Greek historian who had made the opposite assertion to himself or there could be something larger at play.
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