Human factors/usability engineering focuses on the interactions between people and devices. For medical devices, the most important goal of the human factors/usability engineering process is to minimize use-related hazards and risks and then confirm that these efforts were successful and users can use the device safely and effectively.

In my opinion, I would say that the human factors is not required in the case where validation is not required. If no changes were made to the point where clinical trials are not needed and there are no changes to the user interface as you mentioned, then it should not be a requirement. However, I would have to agree that having it would not hurt, it can only help in the long run.