Yeah, the trunks *definitely* get denser as they age. The limbs, such as they are, however, remain really piddly affairs. And if they grow less densely, then the story actually makes even *less* sense. You wouldn't be hearing something that sounded like a beating, but rather a swishy-teary sort of sound as she went through it, and if she was falling at some sort of weird angle... bamboo bends....a full thirty seconds of fall in Cerean gravity still wouldn't get her up to an earth-equivalent speed, but would have given her more than enough time to grab at various bits as she was ....gradually floating faster... towards the ground.
I could be missing something, and it's as cut-and-dried as it's written so far, but Libby's testimony sounds fishy as hell to me, and I bet there's a reason why she's ducking out and trying not to be there in person.