By the strict definition of debt that you employed, of course, HBD is not considered debt. However, we can easily find examples of debt that somewhat resemble what HBD functionally is. For instance, if you open a non-interest-bearing checking account the bank is obligated to pay you back the amount in deposit upon request up to a certain amount, as defined by the FDIC (in the US).

In the case of HBD, the network guarantees that you will receive approximately one US dollar's worth of Hive for every HBD converted, up to a certain amount. That amount is defined by the "debt ratio". In this sense, HBD is debt...kind of.

In my view, HBD is a derivative...plain and simple. The value of that derivative is supported by the collateral that the network is willing to deploy. Someone reading this statement may say that HBD is not the same as DAI (for example), where you put up collateral to mint them.

That is true; however, the network itself uses collateral to support the value of HBD. Even though that collateral is not tracked in any block explorer it is a logical consequence of how the code works.