This appears to either be a sincere product of a socialist Utopian who doesn't understand basic physical processes and tax concepts, or a cleaver work of satire. I'm unsure which at this time.
I think the creator is aware that stock holdings are not legally bound to the income tax rate. That's not the point of the simulation. The point is that the rich have vehicles for their wealth that normal people don't have access to. The point is also that there are pressing, permanent problems in our society that cost less to fix than a quarter of the wealth of one person in our country.