I think it's pretty clear that the purpose of this commercial is to make you believe that eating Cheerios will make you lose weight. But did you catch that last part? "People who eat whole grains tend to weigh less than those who don't."
Ok. Let's think about this. Is eating whole grain the variable that causes people to tend to weigh less? Can you think of any others?
Maybe people who eat whole grain are more conscious of what they eat. Maybe they're more health conscious in general. Maybe people who eat whole grains also tend to work out more. Maybe the average whole grain eater has a faster metabolism. Whole grain items tend to cost more than white flour products, so maybe people who eat whole grain have more money to invest in groceries, weight loss products, gym passes, etc. And note the key word "tend." There is no promise that people who eat whole grain will weigh less than those who don't. The use of the word almost says "there isn't any conclusive evidence but we've noticed that a lot of skinny people eat whole grain."
So. Is eating whole grain, specifically Cheerios, actually going to make you lose weight? I don't know, I've never done a study. However, the fact that they couldn't come out and say that in their commercial makes me think the answer is no. I'm not saying that eating Cheerios is bad for you, I'm just saying you have to be carful when taking in the information that advertisements are giving you.