If I'm not mistaken, in past tech booms, many employees used to become rich by keeping at least some of their stock, though. I think it is somewhat telling if most of the employees (who could be expected to be familiar with the company, its technology, its products and markets) don't seem to expect this to happen here, but rather treat this as a job in a more "mature" industry with little growth potential, such as manufacturing or banking.
Also, capital market investors tend to consider so-called "insider trading" (which includes trades by company employees and executives) as somewhat predictive of stock prices, as far as I know.
I think all of this should be true about almost any other company. However, if OpenAI employees had a reasonably strong belief in the hype surrounding their company and their technology, wouldn't they be holding more shares? After all, the rest of the world is constantly being told that this is the future and that pretty much all of our jobs are at risk because of it.