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

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.