The last sentence of the paper says: ''We took this approach as some extant populations contain a mixture of members of different mtDNA haplogroups and nDNA populations'' - isn't this a suggestion that the populations are admixture and that the few specific haplogroups in some populations are not enough for defining a particular subspecies?
The ''Eastern'' black rhinoceros genetics were more diverse, and diversity decreasing toward endpoints (south-west, south, central, Africa) of distributions of the extant populations in Africa.
That's why I said it would be interesting; I don't know whether the genetic differences are strong enough to warrant separating them as subspecies. As far as I know, there is no baseline standard for delineating subspecies (but if there is, I'd be interested in knowing). I suppose if there isn't, then there would be no need to maintain the current subspecies divisions either.
I don't think the fact that there was admixture is enough on its own to say they are not genetically distinct. For one thing, they say in the paper that park and population management by humans has been the result of some of that mixing. Some is also through "separate" populations being linked together via intermediate populations, creating more of a spectrum than distinct units, but this is also common and doesn't necessarily mean they aren't distinct.