If they were promised equity, a bonus, or some other extra compensation that was not subsequently delivered then not paying that out would be unethical. But agreeing to a lowball amount and then complaining about it being a lowball amount isn't ethics related. It's just sour grapes.
It's the same in the opposite direction. If a contractor charges a company 10x his normal rate and a company accepts those terms, there's no ethics involved either.
Nobody has a proverbial gun to their head to accept a deal either way. If they do then sure there's ethical issues but I haven't seen that in this situation. Just very poor negotiations on the part of the original dev team.