The switch to picamerax seems to have been this commit Switch to picamerax (54c342bb) · Commits · OpenFlexure / openflexure-microscope-server · GitLab ‘11 months ago’. That is about the same time as the thread Support for HQ Pi camera - General - OpenFlexure Forum where it clearly worked as it is there on the screen in the photos, but that could well have been the server version before the change.
But also this PiCameraX - Pi Camera + Extras - Announcements - OpenFlexure Forum says that picamerax should support the Pi HQ camera.
@CarlosZL did it more recently, it is not clear whether using the Openflexure server.