I think liquidity is a key distinction. Many of those Twitter employees are in a lockdown(?) period post IPO where they are not allowed to sell their shares (typically 6 months). Thus they don't have the option of becoming cash millionaires right now. (Many startup founders are this way post series A)

Compare this to a case where someone owns $1MM worth of Google shares, purchased on the open market. Even though they don't have cash, it's basically as if they own cash, since you can liquidate the shares right away.