Sounds like the low density wood is the main reason. It explains why they are very long, very wide and deflexed. They need to be able to hold up to the working stress, which low density woods are not that good at. That is how some native americans made bows from willow, when that is the only available material. Same thing you might want to do if working with pine or fig. The narrow tips is an additional element that would improve the speed, as might one relexed limb, while not losing all the stress reduction durabilty of the other deflexed llimb, I suspect. The buttress roots might also vary in density front top to bottom or have natural curves lending to the design. I doubt they look like stove pipes. But, as the wide limb/narrow tips has been argued by many to the one of the most efficient designs, I think this crazy Andaman shape is the produce of a long tradition of excellent engenering from low quality, but readily available wood.
