It seems that the author is only aware of the existence of general-purpose distributions. They did not list any distribution with, say, different approach to packaging, or for use in HPC, security, NAS, embedded, you name it. Also, for some reason they cannot avoid listing more or less all well established general-purpose distributions, while that is really the list between which you are supposed to choose one or two if you care about not trying many similar things. It seems that the only criterion to be on this list is to be well known. Same for the DEs.