This is incorrect. Currency issued by a central bank is not a real liability, since its issuance does not entail a future payment obligation. It's a liability

This is incorrect. Currency issued by a central bank is not a real liability, since its issuance does not entail a future payment obligation. It's a liability only for accounting purposes. The government bonds on a central bank's balance sheet are used to conduct (or the result of) open market operations. They aren't "backing" anything.