Transmitting and date rates are determined by multiple factors, many that are known. Risk groups, ability and convenience to do social distancing, population density, ventilation, weather, population movement patterns and more. In this case we got Kansas city vs St. Louis, through the article is trying to making an argument about the world in general.
Graphs like this are basically pointless except for one thing. They imply that there might be a benefit of masks, and thus while its not evidence of effectiveness its is not evidence of the opposite. It is what researcher use as support for doing further studies that later accounts for known factors. If however one uses it to jump ahead, someone else can create a similar graph in a different locations and time to create the opposite conclusion.
(Small note about the time period). It is worth looking at the period between April and August. The two lines swap places which each other. St. Louis started out with a higher rate than Kansas city, and end up with a lower rate afterward. This is a pretty common pattern and follow the old "even out the curve" concept.
If I wanted to do the opposite I would pick cities that are inland with temperate climate during that fall, with the air dry and most people inside poorly ventilated buildings.