The function you’re looking at exists on the Polygon Game Object (Phaser.GameObjects.Polygon), but you’re working with the geometric shape (Phaser.Geom.Polygon). The Game Object can be rendered to a scene (just like any other Game Object), while the geometric shape is a logical representation of a polygon.
If you want to use the Game Object, you’ll have to instantiate the correct class (new Phaser.GameObjects.Polygon(scene, coords)) or use the Game Object Factory (scene.add.polygon).
If the geometric shape is actually what you want to use, the smoothing method for it is static - Phaser.Geom.Polygon.Smooth.