I do not know. Believe it or not, I had never noticed this, which is surprising to me. The evidence, as you point out, does not have the Philistines in the Promised Land before about 1200 BC. Most scholars believe that they are some of the "Peoples of the Sea" who invaded and conquered parts of Egypt in the 13th century BC. They probably came from some of the islands in the Mediterranean and were probably influenced by the Minaon civilization, although archaeologists are not sure of the lineage of the Philistines.
Perhaps the one who edited Genesis into its final form changed an earlier word for a people he did not know into a name with which he was familiar. It seems unlikely–indeed to border on impossible–that there were Philistines in Canaan ruling at the time of Abraham. I am forced to conclude that someone who passed on an oral tradition or more likely someone who copied the text tried to "improve" the original by using the name of a people with whom they were familiar. If so, this editor definitely did not improve the text. It is not a secret that there are some copying issues with the Old Testament.
Another thing to consider which supports my contention that there was copyist error (rather than a historical error) is that the group described as "Philistines" in Genesis seems on the face of it to be nothing like the Philistines we find in 1 and 2 Samuel, 1 Chronicles and other books–the Philistines of history. It is not unreasonable at all that a copyist might "correct" a name to the familiar Philistines. What is extremely unlikely is that someone creating a fictional story would put the Philistines in Genesis in what would then be an anachronism. The reason is the the "Philistines" of Genesis are nothing like those of early iron age Canaan. They are friends to the Jews. They are not presented as agressive enemies of Israel. There is no mention of the cities which have always been associated with the Philistines such as Gaza or Ashkelon. If a Jew at a late date was anachronistically inserting the Philistines into some sort of invented history, it seems extremely unlikely that he would create Philistines which were completely different from his own stereotype of this people. Much more likely is that there was a people in Genesis which a later editor/copyist knew nothing of which he, unfortunately, changed to Philistines.