Similarly for Musk, it is easy to imagine how quickly Tesla value would drop if Musk where to tweet "I will now sell all of my shares" (he received a formal complaint because he said something that was vaguely close to it)
This isn't much if an epiphany. These net worth values are just loose estimates of hypothetical values obtained by blindly summing coarse appraisals given the current state of things. They have always been like that.
Leaving aside the bias for figures from the last 200 years (ignoring many figures from earlier epochs, whose shares of global wealth were no doubt comparable, or most likely far higher).