The statement also implies they are making millions every year. But the truth is his tax plan starts raising taxes on people earning $200,000 or more (sometimes he used the figure $250,000). That includes millions of small businesses, many of which struggle to survive after grossing a few hundred thousand dollars a year; and businesses can't be millionaires or billionaires. Only people can.
Moreover, many professional people make a few hundred thousand dollars a year, yet aren't close to becoming either billionaires or millionaires.
Obama's promise is another sham, accomplished by word twisting, at which he is a master. And he gets by with this jiggery-pokery because the media will not call him on it.