The wording is a bit confusing, but I believe they are saying that it would take that long to catch up to Buffett's current wealth. If you assume his assets continue to grow at the same pace as yours, you will never catch up.
Yeah sorry, I was assuming he will die sometime in the next 50-87 years being discussed and his money will be donated to charity. So Hypothetically if he became immortal or decided to give all the money to his children. Since historically the rate of return on capital is higher than the growth in income the gap between people who inherit a lot of capital and wage earners tends to increase over time.