I think the first answer is more accurate. Since he is "retired" most of his income comes from dividends and capital gains, which are taxed at a much lower rate than ordinary income. Also, much of his wealth is in an "Individual Retirement Account" (IRA) which is tax-deferred until you have to start withdrawing money at around age 65. When you do, that money is taxed as ordinary income. So many "tax experts" have opined that he's really not optimizing because he's going to be paying way more tax than he might otherwise have to when he starts withdrawing $$.
Also, his tax returns indicate that he gave more than $3 million to charity last year, so, wow, what a terrible guy.