Could a runner running in the heat who was drinking a sport drink with only potassium chloride and carbohydrates (no sodium at all) get hyponatremia?

Yes. : Any drink that doesn't taste like the ocean is mostly water, regardless of whether it contains sodium or not. Long distance runners can be susceptible to hyponatremia, and risk factors include long distances (marathons, ultra-marathons), women, low body weight, inadequate training, and water loading before and during the race, especially if more weight is gained by water than lost by sweat.