My assumptions:
1- Luffy: His DF grants him some access to fire power, how? Don’t ask me… many theories circulating have explained it.. I’m no scientist
2- Kinemon: I’m 100% sure that Kinemon cannot create fire himself… rather his Swords both of them have been called “Fire swords”… so it’s an ability in the swords just like Enma ability is taking lots of your haki… similarly, the weilder of those swords can create fire with mastery..
3- Zoro: that scene was still never explained fully… but if we go back to the attack… it was never coated with fire… ONLY the wound after getting cut was engulfed with fire, and it wasn’t even regular fire.. it was blue flame… so it’s kinda mysterious.. and it could be more related to Zombie or Ryuuma getting purified than simply Zoro used fire attack…
4- There is a commander in WB crew who can create fire in his sword… I will say it’s just like Kinemon… his sword ability can do that.. or it’s a dial sword like Ohm… or some special ability… but it isn’t like Lunaria