The Hive debt in the case above reffers to the debt of the chain as a whole. All HBD is a debt on top of the HIVE marketcap.
Its not a traditional borrow/lend.
Basicly the chain allows for conversions HIVE<->HBD, expandind the supply. Its very similar to what LUNA-UST is doing, just differnt words. And they dont have a haircut rule (debt limit) that is quite dangerous.
P.S. I cover the haircut rule and the debt here
https://peakd.com/hive-167922/@dalz/the-haircut-rule-the-biggest-risk-for-the-hbd-price-or-historical-data-on-hive-debt-and-hbd-price