It's funny, but have you ever checked what does "non-profit" company actually mean in the US? You would be surprised as it has nothing with an inability of gaining income. I don't remember that fact by heart, but it was something like - until a company doesn't spend more than 50% in total on wages or any benefits for internal team including founders and investors (like salary, bonuses, benefits all kind, dividends etc.) you can have any income and still be eligible for non-profit status.
To be fair it's not all the time the case. The fact that company is not publicly trade doesn't stop people, random investors, business angels, VC and others buy and sell its shares openly with some extra corporate limitations. The difference is only with a company valuation cap and how it was set. I guess that current investors of a Foundation are well aware of a current valuation. At least, for the tax purposes. So probably the author meant that rise in such a valuation that was somehow estimated.