Proper chrome plating, with zinc or nickel base will add about 2-3 pounds more to each wheel. Plus some places cheap out and don't chrome the inner barrels. That place doesn't seem to do that going by what they wrote. And your correct, it does depend on the wheel for sure. My wheels in chrome are 3.3 pounds more each. Plus, going from say 17" wheels to chrome 22's will add alot.Contrary to popular belief chrome plate is a non factor in wheel weight it's more the wheel itself, plating wheels will only add a miniscule amount of weight, driving thru Mickey D's for a Big mac would have more effect
And remember, that is unsprung weight, the worst there is. Adding a few more pounds to your wheels is like adding 75+ pounds to the rest of the car.
Here is some good reading about adding heavy wheels to your car. http://www.miamivalleymiataclub.com/technotes/ChromeWheels.pdf