The article sort of misses the point, and their example illustrates it perfectly. On one instance you're talking about horses with a community for animal lovers. The other, with a community of transportation enthusiasts. There will be some overlap, but I expect the theme of most discussion to be very different. I think having separate communities is actually a good thing. It would be nice to have some way to merge on an individual level if I want to scroll through cute horse pictures and talk about riding to work. Some sort of multireddit type feature might allow that.