These liquidation things are happening when the company does badly. The founders (and the investors) were probably hoping to do a lot better than what they settled for. Half a billion dollar is great, but not so great if you thought you were going for 10bn...
Without knowing the amount of the investment that the two investors made, the multiplier and the participation, and how the company was actually doing, all argument is moot. Who knows how much the investors put on the table in the first place. They took a risk, tried to have their ass covered should things turn bad, everybody agreed. Then shit happened and these guys managed to sell the thing before losing it all.
Also, the founders probably got paid pretty decently in all their founding rounds, so I don't feel too sorry for them...