Interesting articles, though I was surprised to see him say that M. sphacelata ssp. viperina was related to M. beneckei. I know that Crozier's work on DNA placed M. beneckei next to M. sphacelata, but then it also placed M. halei next to Ortegocactus macdougalii, and M. theresae next to Neolloydia conoidea.
Which probably goes to show the difficulty in Mammillaria of determining the "right" set of markers and regions to sample within the DNA. I wouldn't want it just to confirm existing classifications, but these are a bit strange.