I don't really understand this article. First off, for a bond market to be "upside down" that means that the return on investment is less than expected/projected inflation. So yes, it might be upside down, but what matters is a combination of return and security. Frankly nothing looks terribly safe, and beyond that, very little is actually returning anything.
As such, so long as treasuries yield more than stuffing cash in mattresses they are likely to keep their demand high. If there are no "safe" investments that can beat inflation then just beating zero is good enough.