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.
That just means you’re pulling from the lower end of the talent pool. There is nothing wrong with this, but usually talent is correlated with outcome. Most hot startups which are going places are near impossible to get into even for folks with good offers.
Your last sentence is not mutually exclusive with my statement. Both could be simultaneously true. The sheer number of big company employees compared to hot startup makes it hard for everyone good to get into the startup, especially considering they usually have more specific needs. That said it could also be the case that the hot startup cannot easily get good employees from big company.
My point was more that the high end ones they do get are usually in the front piece of the airplane in the acquisition split. Also, the really hot startups are actually paying quite a bit of cash upfront so the original premise of employee sacrifice isn’t as true.
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.