He has been constantly changing his positions and using the proceeds to invest in other companies. The benefit isn't one time, it is compounding over time, because he gets to keep investing the 20% of extra money he would have had to pay in capital gains, over and over again, as he shifts his money between investments. It would pay off anyways even if he did have to pay 20% when he eventually withdraws, but the fact that he gets it all tax-free makes it pay off massively.

He also doesn't have to keep all of his wealth in this Roth IRA, just a percentage. And it clearly is a scalable strategy because lots of people are doing it.