Not quite sure whether this helps in the understanding but so many movies have been shot with mirror reflex cameras and shown in theatres without complaints about accurate focussing. A film motion-picture camera has a revolving mirror shutter unlike a SLR still camera that has it on hinges. The groundglass then is removable/exchangeable, so also needs an exact positioning.
After an adventure with the repair of a Pathé WEBO M, a small gauge movie camera, where prisms and lenses are tucked in with cork pieces and the like, I appreciate everything else. Basically, every camera can be adjusted, if a groundglass image is not in focus at flange focal distance. I mean, at least to me, that is the point of a reflex viewing system. If in doubt, have a camera checked by a pro. And the result given to you on paper