It’s called DailyUI not Daily Product Design. I’m not sure why you expect the challenge to do more than it explicitly says it does?
Product Design is such a vast role that yes, knowing UI is not enough. But it is difficult to grow in so many skills at once. For many, sitting with a specific skill for a little while is what gives them momentum to really grow. Your call shouldn’t be to stop doing the DailyUI challenge, but to make sure you keep learning after that.