One Piece Philippines: Why doesn’t Sanji really use haki?

He does. Just for the most part not the standard and obvious Armament most other people use.According to the SBS (the section in each collected manga volume wherein Oda-sensei responds to reader questions), the Monster Trio each specialise in a different type of Haki. Luffy is a Conquing King user, Zoro is all about Armament, and Sanji is a master of Observation.

Another SBS from before Haki was really explained notes that Diable Jambe doesn’t burn Sanji because “his heart burns hotter”. In other words, he protects his legs with fiery Haki (kind of like how Luffy’s Armament Haki is vulcanised rubber, or Enel’s Mantra was electrified). Further, he’s been doing so at least since Enies Lobby (kind of like how Zoro has been using Observation and Armament since Alabasta). Post-time skip, Sanji relies even more on Haki to make Diable Jambe work, igniting his leg with the heat of his Haki rather than air friction from spinning.