Thanks @cpX – from those threads I draw the conclusion that there is some aspect of OIS that you can't turn off. It's a shame that the link provided in the page that your fourth link leads to, for Fuji discussing this in a support article, is broken.
That will probably be the drive of the Powerzoom.
On my X-T30, with the 16-80mm lens, I hear a constant whirring from the lens while the camera is turned on. I thought this must be the IS, so to conserve battery I went to the menu and set IS to "Shooting+motion", but there was no difference. Then I tried setting IS to "Off" – still no difference, there is still a constant whirring from the lens. I turned the camera off, took the lens off, put it back and turned the camera back on – no difference. What is going on?