I think the premise is flawed simply because the writer says when am I allowed to quit and not be labelled a quitter? Frankly never because if you quit you are a quitter. However, the scenario you postulate could be quitting but it could simply be recognizing that the startup will not achieve it's goals and that it should wind down and return any remaining funds to it's investors (or try to sell itself) that's not quitting it's just being realistic.