What a terrible comparison. South Dakota has one of the lowest population densities in the country. There's very little public transportation, people spend less time in stores and restaurants, and all in all there's just less interaction between people when people are so spread out. Los Angeles County has a very high population density, has wide spread public transportation use, and people interact far more often in places like stores and restaurants.
There's a higher accident rate per 100,000 cars on the roads between 8 AM and 9 AM than between the hours of 10 AM and 11 AM. Why is that? Is it because people who drive between 8 and 9 are worse drivers? No. It's because the roads are more crowded between 8 AM and 9 AM which gives you less room for error. If you're not paying attention at 10:30 when it's less likely there will be someone stopped in front of you, you're less likely to hit someone. If you're not paying attention at 8:30, you're going to run into the back of someone.
That's the case with that comparison. If one person in SD has covid, they're going to come into contact with way fewer people and so will spread it around at a lower rate. If one person in Los Angeles County has covid, they're going to have way more chances to spread it.
Whether masks are effective or not is a different question. I'm just saying that guy is torturing statistics.