That contradicts everything I've heard. Conventional wisdom is that they were a non-Semitic people from Crete or elsewhere in the Mediterranean.
Do you have a source? (I'm just curious; I don't really have an opinion).
...the Philistines were probably of Semitic origin.
MY error. The Philistines were of a non-semitic orgin. My apology.
This is some of what the Bible has to say. As far as whether or not they are Semitic, there seems to be two different views.
The International Standard Bible Encyclopedia presents the case that they are and Aston's Bible Dictionary presents the case they are not.
It is too much to post here especially the ISBE reference.
Hope this helps.
Gen 10:6 And the sons of Ham; Cush, and Mizraim, and Phut, and Canaan.
Gen 10:13 And Mizraim begat Ludim, and Anamim, and Lehabim, and Naphtuhim,
Gen 10:14 And Pathrusim, and Casluhim, (out of whom came Philistim,) and Caphtorim.
1Ch 1:11 And Mizraim begat Ludim, and Anamim, and Lehabim, and Naphtuhim,
1Ch 1:12 And Pathrusim, and Casluhim, (of whom came the Philistines,) and Caphthorim.