This seems to be a case of the proverbial vicious circle. Mobot says Gasteria pseudonigricans is a valid name because it's included in The Plant List, and The Plant List says it's a valid name because it's included in Tropicos, which is the Mobot data system.
In general, The Plant List is not a reliable source of taxonomic information. The latest edition dates back to 2012, and even then the "new" information was largely a recapitulation of the information in the earler edition, with no new taxonomic scrutiny. The Plant List also still lists Gasteria nigricans as an accepted name, for example, although every other taxonomic database classifies it as a synonym.
