I don't think it's a cover for the hubcap. The hubcap covers the center hole in the wheel.I might have misunderstood, but I didn't think his point was concern over the rust. My takeaway was that it's ridiculous that it costs extra to get a cover for the hub.
What happened is some people are taking off the hubcap to expose the rims. Now they need to cap the center, and that cover is not standard with your $100k purchase.