You're right of course. The authors of this study even mention the limitations of their own study. They pointed out the subjects in the study were well-trained and used to heavy loads on the trunk. They aren't trying to use the results of their study to make conclusions about the general adult population.
A study of this type can still help nurses. Judo fighters rarely have back problems. If researchers can map out how they move and use their muscles, then maybe it will be possible to help train anyone lifting heavy loads how to lift and avoid injuries.
The study only included 10 subjects, so the size of the sample was small, too. More studies are needed, but scientists have to start somewhere!