Mr. Bloomberg's privately held equity undoubtedly makes distributions to its shareholders. Stock that does not pay dividends and is unlikely ever to do so is not wealth; it's just a piece of paper. And if you cannot legally sell it to someone else, it's effectively worthless as there is no way to convert it into something of value. The exception is if you have a majority of the voting rights, but that's not what this article is about.
There are many liquidity events that are not in the set of 'IPO' events. But the issue remains that liquidity is a core element of asset value. Many later stage financing events can be structured to provide limited liquidity to founders and early investors; the issue is that this may not help all employees equally. The IPO is useful in this case.
