Read the article, it clearly says... On platforms which do not support “-apple-system,” the browser will simply fall back to the next item in the font-family f

Read the article, it clearly says... On platforms which do not support “-apple-system,” the browser will simply fall back to the next item in the font-family fallback list. This provides a great way to make sure all your users get a great experience, regardless of which platform they are using. There are currently discussions in the w3c regarding standardizing this value so authors could simply specify “system.” However, these discussions have not reached consensus, so WebKit prefixes this value.