In my opinion, that was a wrong call. I've done it for back-to-backs, too, as well as through studs between panels.... about 15 years ago the local AHJ made me replace a 6" piece of 2" running thread that I had used on a service upgrade between the exterior 200a meter pan and interior panel board. No couplings, just 4 locknuts and 2 bond bushings. Had to replace it a grc nipple or pvc.
...just 4 locknuts and 2 bond bushings...