IMO this is a PR move, meant to garner publicity, but not really workable.
The sun changes positions throughout the year, then there are clouds, it doesn't make sense.
> As a result of its creative promotion, Emart reportedly saw membership increase by 58 percent in February over the previous month, they also observed a 25 percent increase in sales during lunch hours.
Yeah, so they pretty much come out and say it's a PR move. And is workable, apparently. As for the issues you suggest:
While this is true and will most likely not work for all businesses attempting to gain publicity, they could always pivot the QR code to adjust for the sun's position and/or clouds.
