Wait wait, let's game this out. No rule says you have to be X distance from cover, does it? And there are some benefits to spacing yourself futher rather than closer to your cover as you engage targets. So in this case he backed away from the cover, call it a wall corner for example. Say you back off 8 feet from the wall, you slice the pie correctly, engaging all targets as they become visible and you neutralize them. As long as you have not exposed yourself to any other targets anywhere, what rules have you broken? You may end up 8' out and away from the end of the wall depending on where the targets are.
Picture a bicycle wheel. The end of the wall is the axle, and the farther you go along out a spoke towards the rim the more you will move as you rotate around the axle. More linear distance but the number of degrees remain the same.
This may take some rules studying and pondering. It may be a legit idea not disallowed in the rules. In a strictly gamer sense anyways.