That argument is both true and deceiving at the same time. The poorest countries are improving the most because they are so far behind everyone else that any improvement is spectacular when viewed as a percentage.
Take a homeless person who has a penny to his name. You give him $100 and his wealth skyrockets by 1000000% in a day. Compare that to bill gates whose wealth probably increases by a "measly" 10% per year. But if you look at the wealth increase by absolute numbers rather than percentages, the picture is quite different. Bill Gates' 10% probably gets him $10 billion while the homeless man's 1000000% gets him $100.
$10 billion vs $100.
10% vs 1000000%.
Whereas the mainstream media may be too pessimistic, rosling is even more optimistic. The idea that the poorest countries ( like CAR ) is going to lead the world is unrealistic barring some apocalyptic event in the US, Europe, East Asia, Middle East.