It’s rooted in the term “Philistia” from the Romans and it included essentially the entire Gaza strip.
Insofar as its exclusivity, the so-call Palestinians took it up as a national front identification from the early sixties.
They’re still nothing but a bunch of disaffected, inbred islamist arabs to me.
No matter how it came about, no matter what the original reason was and no matter whether you like it or not in this 21st century ... it is thoroughly rooted in history, and thoroughly rooted in the writings of the time period during which the Jews started returning in large numbers to the land (mid-1800s to present) and thoroughly rooted in the documents that Israel uses for their legal justification in the modern world ... and ... the Israelis use it everyday in all their normal conversations without any qualms as to its usage.
You can consider them anything you want ... but it doesn’t change the usage of the term.