The implicit promise is only partially true. Very rarely you can find a proven talent that will actually forego significant salary. Often time when that happens the person is close to founders and will have a significant role in shaping the startup and will get quasi-acquired too.

This promise may have been more true before 2010s where public companies were not paying as much in liquid cash and private companies were not valued so aggressively. Fact is most employees take the startup offer because they don't actually have a liquid offer that's super competitive at that moment, or they are just kind of bored and taking a break of the corporate job that does not give them too many responsibilities, i.e. they are compensated via the title, not just the promise of making bank.