First of all, that passage specifically limits the definition of that debt, despite the massive bogusity (it's a technical term) of Perry v. United States (1935).
But second of all, what it actually refers to is the legitimacy of the debt to the Federal Reserve. In other words, the Constitution authorizes the government to be the source of our money, and Congress then passed this responsibility on to a private corporation, the Fed, to print the money and loan it back at it's printed value - a scam of such unbelievable proportions, most people... don't believe it. But it's true, and such a wildly hyperbolic violation of the Constitution, that they decided to say, in the 14th Amendment that... "hey, you know that giant ripoff of our currency we made to the Fed through totally artificial, wildly inflated debt? Well, you hereby cannot question whether that debt is valid."
It's like declaring that people have to be taxed when the sky is red, and then passing a law that the blue sky is red, and then passing an Amendment that says "the validity of the definition of the sky as red shall not be challenged."
No, on the other hand, it's not "like" that.
It IS that.
The 14th Amendment was passed in 1866 to prevent the Confederate States from trying to evade paying their share of the debt incurred by the Union in fighting the Civil War.
The Federal Reserve Bank was not created until 1913. The 14th Amendment had been in place for almost 50 years before the Federal Reserve and therefor could not possibly refer to any debts owed to it.