I do not know the details, but the RHS apparently treats the two species as interchangeable, while the Catalogue of Life (and our database) treats them as distinct. Interestingly our database considers the named cultivar to be a form of Delosperma basuticum.
Ice Plant (Delosperma basuticum 'Gold Nugget')
And apparently this cultivar has a couple of offspring. I don't know any more than this.