I believe it is more likely that the color was among the founding stock brought to the country, and not slipped in later. Argentina was a big importer of colorful stock, back before stud books were common. There were British Hackney breeders who raised tobianos specifically for that market. Some made it into the early Hackney stud books, and one was exported to the US and went on to be a foundation horse for the Moroccan Spotted Horse, but many more were said to be sent to Argentina. It is not hard to believe that other colorful animals went there, too, and that remnants in the form of a hidden recessive remained after solid colors became more fashionable. In that way, the breeders may never have knowingly included this. All it would take is that they use local mares in the early stages of their purebred breeding programs, and it could from there become a part of each domestic breeding group there.