I also came across something in the book The Exodus by Richard Elliott Friedman. He mentions the steles bearing evidence of Israel, and notes that they speak of Israel being no more, that its seed is wiped out. We know this not to be true, and he compares it to language such as “we killed 'em” and “we slaughtered 'em.”
In addition, note that the book of Joshua proclaims that the Canaanites are wiped out. Yet we know from Judges this is not so, and even from the text of Joshua. Look at the story of Rahab. Now one may take a critical approach and say Judges was written by another so it differs, but as Catholics we must understand the text as a coherent whole. Therefore, I propose the following, which I am not claiming to be new: the language of total extermination in the text of Joshua is hyperbolic in the vein of the steles I mentioned above. Just a way of speaking back in those days. Consequently, the order may have been similarly hyperbolic, either in original form or as it was transmitted down to us.
It gets more interesting when one gets into mystical explanations. Some saint, I believe, proposed that the story was symbolic of the seven deadly sins being conquered. This fits into the evidence that there was no large scale conquest of the land and that the Israelites arose relatively peacefully in Canaan itself. The conquest then becomes symbolic. However I have no opinion on whether the conquest happened or not. Kenneth Kitchen, a respected historian, contends it did. Others don’t. I have no idea. Just throwing this out there.