That website appears to be some third party, and is not the IRS or the U.S. gov't.
PayPal can be used for the same thing. If Paypal got hacked, and you couldn't use it to pay taxes on it, would that get described as having "taken out the US Federal Tax Payment System"?
Plus, even if this was the IRS's systems taken out ... why would the Russians be willing to do such a favor for the everyday, ordinary American?