It's meant to be ironic, making the point that people don't really change. Not sure what would be offensive about it unless you were a proud descendant of the actual Philistines, and objected to the use, since the 1600s, of the term philistine (uncapitalized, as this one probably should have been) to mean an uncultured boor.

need I be a member of a group to find a derogatory term for that group offensive?

I suppose not, but that would make you a... good Samaritan.

ok. ok. good one.

I don't find it offensive but using demonyms as insults has always seemed distasteful to me, even if the group in question no longer exists.