Getting screwed out of your payout by such a totally-not-an-acquisition is wage theft. It's like promising a sales-related bonus at the beginning of the year, and then in December changing the metric to "AI-related sales to the CEO's golf buddies".
I understand that a lot of inexperienced people (like in this thread) think they're going to get rich though.
No, it is not "wage theft" to not get rich when the company exits (by whatever means).
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.