"And I know this for a fact. It came straight from the muthafukka who lives 2 houses down. He comes straight from Zamboanga City. Born and rasied over there. 100% full blooded Flip."

your neighbor probably just migrated to the US and is clueless about it. heck there are some guys whom you can call whatever you want but still smile at you just so they can stay out of trouble or "fit in". zamboanga is also a province in the philippines very far from metro manila. the chances of him meeting fellow zamboanguenos who experienced living in the US is far less as compared to people who lives in metro manila. so he might not be educated about slurs in the US.
its been explained to you alot of times already InTheNeutralCorner also posted a link proving its a slur. and if you use that word again then your probably racist. and is intentionally hating on filipinos.