I agree with Pug. When I use a weight belt, both are right hand release for me as well. As stated on the video, the buckle on the harness should be positioned far to the right of center on your body so that if you need to ditch, and bring your hand to your waist it won't be anywhere near the buckle for your harness. This is to hold the canister light in place and also to prevent it from being mistaken for the weight belt buckle, which should be in or near the center of your body.
I haven't taken the DIR-f course yet, largely due to circumstance more than deliberately putting it off. From the reading and research that I have done, however, this is the justification that I have seen.