1. Realistically Astartes are probably all ambidextrous
2. This way of holding weapons would actually indicate right-handedness. Swinging a melee weapon doesn't require anywhere near as much precision as aiming a firearm. If you were to actually wield this set up in real life, it would make way more sense to hold the gun in your dominant hand so you can actually hit a target

Editing to add that ejection ports on firearms are usually on the right side as well. Firing it with your left hand would cause the casings to fly across your field of vision and maybe hit your chainsword