Stop me if you heard this one before. The triple-A credit of the U.S. is threatened by the nation’s burgeoning budget deficit. Should bond investors beware? Yes, but not for the reasons the green-eyeshade types at the bond ratings firms think.

The passage of the unprecedentedly huge coronavirus crisis bill, totaling more than $2 trillion, by the Senate—complementing the Federal Reserve pulling out all the stops to shore up the financial system—appeared to halt the downward spiral of the stock and credit markets. While that...