From https://aphascience.blog.gov.uk/2016/11/18/colistin-an-antib...
"Colistin is considered a drug of last resort in human medicine" but also "It has been used in Europe for decades to treat intestinal infections in farm animals, although in the UK, it has only more recently been used to treat infections in animals"
The Guardian article paints a picture of developing countries irresposibly using colistin for animals, but I dont think that is the full story here.