I tend to think that they can make the guarantee. In the Uinted States we have the following:
A certificate of deposit or CD is a time deposit, a financial product commonly offered to consumers by banks, thrift institutions, and credit unions.
Such CDs are similar to savings accounts in that they are insured and thus virtually risk-free; they are "money in the bank." (They are insured by the FDIC for banks or by the NCUA for credit unions.) They are different from savings accounts in that the CD has a specific, fixed term (often three months, six months, or one to five years), and, usually, a fixed interest rate. It is intended that the CD be held until maturity, at which time the money may be withdrawn together with the accrued interest. While Domestic CDs are USD denominated CDs, Yankee CDs (YCDs) are USD denominated CDs issued by the US branch of a foreign bank.
And then we have savings bond. Savings bonds are treasury securities for individual investors. US Savings Bonds are a registered, non-callable bond issued by the U.S. Government, and are backed by its full faith and credit.
And now we have the PDR. So I do think they can make this guarantee. Our government does it. JMHO :)