No matter how many times you say that this situation is "literally the definition of debt" you're still not going to the legal definition of debt so that it applies here. Equity is not debt, they do both until eventually owing another entity money. But in a legal sense that is not literally the definition of debt, because it is also part of equity which again isn't debt.

From a practical perspective the difference is that with equity you accept a lower floor (you might get nothing) in exchange for a higher ceiling (your invest might 100x if the company goes public). That's the deal these investors signed up for and unfortunately for them they got option number 1. They're not owed anything because that's the deal they agreed to.