A Roth IRA is beneficial if positions are being changed (taxable events), because those taxes are deferred. For very long term holding, the benefit caps out at 20% in exchange for tying up the assets for decades.
He will eventually benefit, if the company is still valuable when he comes of retirement age, by avoiding the single long term capital gains hit he would otherwise pay. It'll be more of a story, then, if the stock is near an all time high value, but frankly, this is not a scalable investment strategy for the wealthy sheltering their wealth (which would be a more interesting story). This only worked because he has been an exceptionally prescient investor. This wouldn't be a good strategy for every startup founder, because even if the startup was mega-successful, the founders would not be rich until retirement.