Oh interesting. From my last deep dive into this subject (admittedly from quite a long time ago) they had only established that significant kava usage showed elevated liver enzymes that they inferred to be an indication that kava causes liver damage but no solid proof was established that directly implicated kava consumption as causing liver damage.

From memory the study was providing people with whole powdered kava in capsules (for obvious reasons) and it was speculated that this may have been the cause for the elevated enzymes whereas the traditional water extraction does not as regular kava users, especially in cultures that consume kava, have not reported showing elevated liver enzymes.

Whether that's true or simply a product of the fact that some Pacific Islands populations are quite poor and lack access to high quality healthcare where they'd notice this phenomenon, or whether it's simply a known thing in their hospitals that most people in the region show elevated liver enzymes upon testing so they disregard it is a matter up for debate.