@HaxeyeMihawk:
Look my point is that it was clearly shown that Sandersonia and Marigold have different haki techniques than one another which bear striking resemblance to other techniques who's origins were never fully explained either(Mantra, tekkai). To say that Sandersonia got burned as a plot device is clearly ignoring details of what was shown and is assuming that Oda would do something so hackish and if that's what anyone is implying then there's nothing to get. And again claiming that Marigold's snake scales are simply fire resistant when she was shown using haki for everything else is ignoring details too.
Look, it's clear that haki can be used for different techniques, as was seen by the boa sisters, but something like hair being used as a weapon or a person setting themselves on fire, it's not unlike the many strange powers we have seen from day one that people can do without devil fruits. How can you hypnotise Luffy by painting something on his back (which he can't even see), Wanze's ramen techniques, etc. Luffy's superhuman strength was even indicated to explicitly NOT be haki related, so we know all these powers that don't exist in the real world, can just happen in the 'Odaverse'.
And above all, Oda is telling a story, not just stringing together an encyclopedia of special powers, so when one boa sister sets the other on fire to set the stage for Luffy to do a heroic rescue, there's no need to look at it any different or suddenly tie it with all the superhuman things we've had people do in the past.
Before the Amazon Lilly arc, when we were formally introduced to haki, its usage was always indicated by some kind of 'eye' power. Shanks, Rayleigh in the auction house, Sentoumaru, even Luffy when he used it without knowing on that bull. Oda even made a point to indicate something out of the ordinary was going on - think about the comments when Sentoumaru hits Luffy, "why can he do damage on a rubber body", even though plenty of people have been doing this to Luffy with just blows. So even though it shouldn't be that strange, still Oda made a point to show something peculiar was up with this technique. That's a pretty clear indicator that he's introducing something new, and not something people have been constantly using all along!