If anything, this strengthens the argument by eliminating differences in sitting time at home as a potential explanation. For example, if office workers had sit a lot more at home, then the overall sitting time might have been the true explanation for the higher rate of injury.

What insurance companies prefer has no impact on what might explain the disparity in lower-back injury between office workers and physical laborers.

We have no reason to think office workers aren’t similarly encouraged to use techniques that reduce back stress (ex. seat cushions). In addition, even if the laborers can reduce back stress, we’d still expect them to get back injuries at a higher rate than office workers.

The timing of injuries doesn’t have clear impact on the argument. Back stress might cause injury at work or it might build up and lead to injury at home. If this answer does anything, it might strengthen the argument by connecting injuries to the office.

Laborers get much more physical exercise (which includes more than just gym activities/cardio) than office workers on the job. This disparity in physical exercise, rather than office equipment design, could be the true explanation for the disparity in lower-back injuries.