If your legal tender currency is created primarily through loans, then it is loan-backed.
As you noted, the concept of a "loan" entails the possibility of default.
If most currency is created through loans and loans must be repaid in that currency, the system-wide probability of defaults is significant. The likelihood of defaults increases as loans require payors to also include tribute payments (interest), for which no currency was ever created (necessitating that the payor obtain the requisite currency from someone else before the default day occurs).
You could have a default on a loan if you were paying for your house with NukaCola bottle-caps, or engraved-crab-shells.