Because all primitive types duck-type to each other by default. An int is a double is a string is an array is an object.
Duck typing is basically doing the work of adding interfaces to your code so you don't have to think about it. That's all. You can implement it in static languages with a *-ton of interfaces,
Duck-typing can be done automatically by an IDE and can be built into a language. Duck-typing is not a feature of or the sole preserve of dynamic languages. Stop quoting it as such.