That's a gross exaggeration. If you own 0.1% of the company, you're likely to be diluted to around 0.02% at the very worst, and most likely 0.04% or so assuming 3 rounds of funding.
That's assuming a positive outcome that clears preferences of course - if the company goes in a fire sale you're not going to make anything at all. But assuming a billion dollar exit means you're assuming not a fire sale.
I was being flippant in my use of decimals there, but that's a detail.
Even if you get 0.1% and are ultimately diluted to 0.01%, a billion dollar sale is $100k. When people sign up for below-market pay, they're not envisioning a distant eventual windfall of a year of market-rate salary.
So, if we take the above poster at their word that it would have to be a billion dollar exit to be worth their while (and net them a million dollars for X years at below market salary) and assume your dilution numbers are accurate, they'd only get 200 to 400k. There would have to be a 2.5 to 5 billion dollar exit for their diluted tenth of a percent to net them a million.
I'm taking the post we're responding to at face value: "If I'm going to take less than what I believe to be the "market rate" for my services in lieu of some equity and my motivation is to make money, then I'm going to do the math and weigh the probabilities of my equity and the lower-than-market salary being more lucrative than taking a job with no equity and a market rate salary."
It depends on when you get hired. Early engineering key hires, pre series A, are often 0.5-3%, but is probably around 50% of market salary ($80k on $160k, say). Employee 100 in the same role in a highly successful company could be as low as 0.01%, but is nearly at market salary ($140k?). There might be 2-4 years in between the two hires.
